Glossary

ABO blood group A set of four phenotypes produced by different combinations of three alleles at a single locus; blood types are A, B, AB, and O.

abscisic acid (ABA) A plant hormone controlling, among other things, environmental stress responses, and seed and bud dormancy.

abscission In vascular plants, the dropping of leaves, flowers, fruits, or stems at the end of the growing season.

absorption spectrum The relationship of absorbance vs. wavelength for a pigment molecule.

acceptor stem The 3' end of a tRNA molecule; the portion that amino acids become attached to during charging.

accessory pigment A secondary light-absorbing pigment used in photosynthesis, including chlorophyll b and carotenoids.

acetyl-CoA Metabolic intermediate consisting of an acetyl group linked to coenzyme A.

achiasmate segregation The accurate segregation of homologues during meiosis I without the formation of chiasmata.

acid Any substance that dissociates in water to increase the hydrogen ion (H+) concentration.

acoelomate An animal, such as a flatworm, having a body plan that has no body cavity.

acquired immunodeficiency syndrome (AIDS) An infectious disease caused by HIV, which infects helper T cells.

actin One of the two major proteins that make up vertebrate muscle.

actin filaments One of the three elements that make up the cytoskeleton; also called microfilaments.

action potential A transient, all-or-none reversal of the electric potential across a membrane.

action spectrum A measure of the efficiency of different wavelengths of light for photosynthesis.

activation energy The energy that must be processed by a molecule in order for it to undergo a specific chemical reaction.

active site The region of an enzyme surface to which a specific set of substrates binds.

active transport The pumping of ions or other molecules across a cellular membrane against a concentration gradient; requires energy.

adaptation A peculiarity of structure, physiology, or behavior that promotes survival and reproduction.

adapter protein Any of a class of proteins that acts as a link between a receptor and other proteins.

adaptive radiation The evolution of several divergent forms from a primitive and unspecialized ancestor.

adenosine triphosphate (ATP) The energy currency of cellular metabolism in all organisms.

adenylyl cyclase An enzyme that can be activated by GPCR signaling to convert ATP to cyclic AMP.

adherens junction An anchoring junction that connects the actin filaments of one cell with those of adjacent cells.

adhesion The tendency of water to cling to other polar compounds due to hydrogen bonding.

adipose cells Fat cells, found in loose connective tissue, that store droplets of fat.

adventitious Referring to a structure arising from an unusual place, such as stems from roots.

aerenchyma In plants, loose parenchymal tissue with large air spaces, often found in aquatic plants.

aerobic Requiring free oxygen; any biological process that can occur in the presence of gaseous oxygen.

aerobic respiration Process resulting in the complete oxidation of glucose using oxygen as the final electron acceptor.

aleurone In plants, the outer layer of the endosperm in a seed.

alga A unicellular or simple multicellular photosynthetic organism lacking multicellular sex organs.

allantois A membrane of the amniotic egg that functions in respiration and excretion.

allele One of two or more alternative states of a gene.

allele frequency A measure of the occurrence of an allele in a population.

allelopathy The release of a substance from the roots of one plant that blocks the germination of nearby seeds.

allometric growth A pattern of growth in which different components grow at different rates.

allopatric speciation The differentiation of geographically isolated populations into distinct species.

allopolyploid A polyploid organism that contains the genomes of two or more different species.

allosteric activator A substance that binds to an enzyme’s allosteric site and keeps the enzyme in its active configuration.

allosteric inhibitor A noncompetitive inhibitor that binds to an enzyme’s allosteric site.

allosteric site A part of an enzyme, away from its active site, that serves as an on/off switch.

alpha (α) helix A form of secondary structure in proteins where the polypeptide chain is wound into a spiral.

alternation of generations A reproductive cycle in which a haploid phase gives rise to gametes, which fuse to form a diploid phase.

alternative splicing In eukaryotes, the production of different mRNAs from a single primary transcript.

altruism Self-sacrifice for the benefit of others.

alveolus One of many small, thin-walled air sacs within the lungs.

amino acid The subunit structure from which proteins are produced.

aminoacyl-tRNA synthetase Enzymes that attach specific amino acids to the correct tRNA.

amniocentesis Indirect examination of a fetus by tests on cell cultures grown from fetal cells in amniotic fluid.

amnion The innermost of the extraembryonic membranes; forms a fluid-filled sac.

amniote A vertebrate that produces an egg surrounded by four membranes.

amniotic egg An egg isolated and protected from the environment by a shell.

ampulla In echinoderms, a muscular sac at the base of a tube foot.

amyloplast A plant organelle called a plastid that specializes in storing starch.

anabolism The biosynthetic or constructive part of metabolism.

anaerobic Any process that can occur without oxygen.

anaerobic respiration Use of electron transport to generate a proton gradient for chemiosmotic synthesis of ATP without oxygen.

analogous Structures that are similar in function but different in evolutionary origin.

anaphase The stage of mitosis and meiosis II initiated by the separation of sister chromatids.

anaphase-promoting complex (APC) A protein complex that triggers anaphase by degrading cohesin.

ancestral similarity Similarity among species inherited from the most recent common ancestor.

anchoring junction Cell junction that mechanically attaches the cytoskeleton of a cell to adjacent cells.

androecium The floral whorl that comprises the stamens.

aneuploidy The condition in an organism whose cells have lost or gained a chromosome.

angiosperms The flowering plants; ovules are completely enclosed by tissues at pollination.

animal pole The hemisphere of the blastula comprising cells relatively poor in yolk.

anion A negatively charged ion.

annotation In genomics, the process of identifying landmarks in a DNA sequence.

anonymous markers Genetic markers that do not cause a detectable phenotype.

antenna complex A complex of hundreds of pigment molecules in a photosystem.

anther In angiosperm flowers, the pollen-bearing portion of a stamen.

antheridium A sperm-producing organ.

anthropoid Member of the mammalian group consisting of monkeys, apes, and humans.

antibody A protein called immunoglobulin produced by lymphocytes.

anticodon The three-nucleotide sequence at the end of a tRNA molecule complementary to an mRNA codon.

antidiuretic hormone (ADH) A pituitary hormone that stimulates the kidneys to reabsorb water.

antigen A foreign substance that stimulates an immune response.

antigenic drift Genetic change in RNA viruses due to random mutation.

antigenic shift Genetic change in RNA viruses due to shuffling of genomic segments.

antiporter A carrier protein that transports two molecules in opposite directions.

anus The terminal opening of the gut.

aorta The major artery of vertebrate systemic blood circulation.

apical ectodermal ridge Region of a developing vertebrate limb bud responsible for growth.

apical meristem In vascular plants, the growing point at the tip of the root or stem.

apoplast route In plant roots, the pathway for movement of water and minerals through cell walls.

apoptosis A process of programmed cell death.

aposematic coloration Ecological strategy where organisms advertise poisonous nature by bright colors.

aquaporin A membrane channel that allows water to cross easily.

aquifers Permeable, saturated, underground layers of rock, sand, and gravel.

archegonium Multicellular egg-producing organ in bryophytes and some vascular plants.

archenteron The principal cavity of a vertebrate embryo in the gastrula stage.

arteriole A smaller artery leading from the arteries to the capillaries.

artificial selection Change in the genetic structure of populations due to selective breeding by humans.

ascomycetes Part of the "true fungi" characterized by separate hyphae and ascospores.

ascus Specialized cell in ascomycetes where two haploid nuclei fuse to produce a zygote.

asexual reproduction Inheriting all chromosomes from a single parent; cell division is by mitosis.

A site In a ribosome, the aminoacyl site which binds to tRNA.

assortative mating Type of nonrandom mating where phenotypically similar individuals mate more frequently.

aster A radial array of microtubules in animal cell mitosis.

astrocytes Neuroglial cells that play a role in maintaining the blood-brain barrier.

atom The smallest unit of an element that contains all its characteristics.

ATP synthase Enzyme responsible for producing ATP in oxidative phosphorylation.

atrial peptide Small polypeptide hormones used in treatment of high blood pressure.

atrioventricular (AV) node Connection of cardiac muscle cells that receives heartbeat impulses.

atrium An antechamber in the heart or ear.

autonomic nervous system Involuntary neurons that regulate the heart, glands, and smooth muscle.

autopolyploid A polyploid organism that contains a duplicated genome of the same species.

autosome Any eukaryotic chromosome that is not a sex chromosome.

autotroph An organism able to build all the complex organic molecules it requires.

auxin A plant hormone that controls cell elongation.

auxotroph A mutation that affects a biochemical pathway causing a nutritional requirement.

avirulent pathogen A pathogen that utilizes host resources but does not cause extensive damage.

axil The angle between a leaf’s petiole and the stem.

axillary bud A bud found in the axil of a stem and leaf.

axon A process extending out from a neuron that conducts impulses away.

β oxidation Oxygen-dependent reactions where fatty acids are cleaved and combined with CoA.

β sheet A form of secondary structure in proteins forming a planar structure.

b6–f complex See cytochrome b6–f complex.

bacteriophage A virus that infects bacterial cells; also called a phage.

Barr body A condensed and inactivated X chromosome seen in interphase nuclei.

basal body A self-reproducing, cylindrical cytoplasmic organelle.

base Any substance that dissociates in water to absorb hydrogen ions.

base-pair A complementary pair of nucleotide bases.

basidium Specialized reproductive cell of basidiomycetes, often club-shaped.

basophil A leukocyte containing granules that enhance the inflammatory response.

Batesian mimicry Survival strategy where a palatable organism resembles a toxic one.

B cell A type of lymphocyte capable of secreting a specific antibody protein.

behavioral ecology The study of how natural selection shapes behavior.

bicoid A gene in Drosophila involved in anterior–posterior specification.

biennial A plant that normally requires two growing seasons to complete its life cycle.

bilateral symmetry A single plane divides an organism into two structural mirror images.

bile salts Organic salts secreted by the liver that emulsify fats.

binary fission Asexual reproduction by division of one cell into two equal parts.

binomial name Scientific name of a species consisting of genus and species.

biochemical pathway A sequence of chemical reactions where products become substrates for the next.

biodiversity The number of species and their range of adaptations in an area.

biofilm A complex bacterial community comprising different species.

biofuel Fuel produced via biological rather than geological processes.

biogeography The study of the geographic distribution of species.

biological community All the populations of different species living together in one place.

biological species concept (BSC) Concept defining species as groups of populations that can interbreed.

biomass The total mass of all living organisms in a given area.

biome One of the major terrestrial ecosystems.

bipolar cell Specialized neuron connecting cone cells to ganglion cells in the visual system.

biramous Two-branched; describes the appendages of crustaceans.

bithorax complex A cluster of homeotic genes in Drosophila.

blade The broad, expanded part of a leaf.

blastocoel The central cavity of the blastula stage.

blastodisc A disclike area on the surface of a large, yolky egg.

blastomere One of the cells of a blastula.

blastopore Opening that connects the archenteron cavity with the outside.

blastula An early embryonic stage consisting of a hollow, fluid-filled ball of cells.

Bohr effect Release of oxygen by hemoglobin in response to elevated CO2.

book lung Respiratory system in some spiders consisting of leaflike plates.

bottleneck effect Loss of genetic variability when a population is reduced drastically.

Bowman’s capsule The bulbous unit of the nephron which surrounds the glomerulus.

bronchus One of a pair of respiratory tubes branching into either lung.

bud An asexually produced outgrowth that develops into a new individual.

buffer A substance that resists changes in pH.

C3 photosynthesis The main cycle of the dark reactions of photosynthesis, in which CO2 binds to ribulose 1,5-bisphosphate (RuBP) to form two 3-carbon phosphoglycerate (PGA) molecules.

C4 photosynthesis A process of CO2 fixation in photosynthesis by which the first product is the 4-carbon oxaloacetate molecule.

cadherin One of a large group of transmembrane proteins that contain a Ca2+-mediated binding between cells; these proteins are responsible for cell-to-cell adhesion between cells of the same type.

callus Undifferentiated tissue; a term used in tissue culture, grafting, and wound healing.

Calvin cycle The dark reactions of C3 photosynthesis; also called the Calvin–Benson cycle.

calyx The sepals collectively; the outermost flower whorl.

Cambrian explosion The huge increase in animal diversity that occurred at the beginning of the Cambrian period.

CAM plant Plants that use C4 carbon fixation at night, then use the stored malate to generate CO2 during the day to minimize dessication.

cAMP response protein (CRP) See catabolite activator protein (CAP).

cancer The unrestrained growth and division of cells; it results from a failure of cell division control.

capillary The smallest of the blood vessels; the very thin walls of capillaries are permeable to many molecules, and exchanges between blood and the tissues occur across them.

capsid The outermost protein covering of a virus.

capsule In bacteria, a gelatinous layer surrounding the cell wall.

carapace Shieldlike plate covering the cephalothorax of decapod crustaceans; the dorsal part of the shell of a turtle.

carbohydrate An organic compound consisting of a chain or ring of carbon atoms to which hydrogen and oxygen atoms are attached in a ratio of approximately 2:1.

carbon fixation The conversion of CO2 into organic compounds during photosynthesis; the first stage of the dark reactions of photosynthesis.

carotenoid Any of a group of accessory pigments found in plants; in addition to absorbing light energy, these pigments act as antioxidants.

carpel A leaflike organ in angiosperms that encloses one or more ovules.

carrier protein A membrane protein that binds to a specific molecule that cannot cross the membrane and allows passage through the membrane.

carrying capacity The maximum population size that a habitat can support.

cartilage A connective tissue in skeletons of vertebrates. Cartilage forms much of the skeleton of embryos and some adult vertebrates, such as sharks.

Cas9 (CRISPR-associated protein 9) A nuclease associated with the CRISPR system of bacterial immunity. These have been modified for genetic manipulation to allow editing of DNA sequences.

Casparian strip In plants, a band that encircles the cell wall of root endodermal cells, forming a layer through which water cannot pass.

catabolism In a cell, those metabolic reactions that result in the breakdown of complex molecules into simpler compounds, often with the release of energy.

catabolite activator protein (CAP) A protein that, when bound to cAMP, can bind to DNA and activate transcription. Also called cAMP response protein (CRP).

catalysis The process by which chemical subunits of larger organic molecules are held and positioned by enzymes that stress their chemical bonds.

catecholamine A class of hormones secreted by the adrenal medulla that are synthesized by a modification of the amino acid tyrosine.

cation A positively charged ion.

cavitation In plants and animals, the blockage of a vessel by an air bubble that breaks the cohesion of the solution in the vessel.

CD4+ cell A subtype of helper T cell that is identified by the presence of the CD4 protein on its surface; targeted by the HIV virus.

cecum In vertebrates, a blind pouch at the beginning of the large intestine.

cell cycle The repeating sequence of growth and division through which cells pass each generation.

cell determination The molecular “decision” process by which a cell becomes destined for a particular developmental pathway.

cell-mediated immunity Arm of the adaptive immune system mediated by T cells, which includes cytotoxic cells.

cell plate The structure that forms at the equator of the spindle during early telophase in the dividing cells of plants and a few green algae.

cell-surface marker A glycoprotein or glycolipid on the outer surface of a cell’s membrane that acts as an identifier.

cell-surface receptor A cell surface protein that binds a signal molecule and converts the extracellular signal into an intracellular one.

cellular blastoderm In insect embryonic development, the stage during which the nuclei of the syncytial blastoderm become separate cells.

cellular respiration The metabolic harvesting of energy by oxidation, ultimately dependent on molecular oxygen.

cellulose The chief constituent of the cell wall in all green plants and some algae; an insoluble complex carbohydrate formed of glucose molecules.

cell wall The rigid, outermost layer of the cells of plants, some protists, and most bacteria.

central nervous system (CNS) The portion of the nervous system where most association occurs; composed of the brain and spinal cord.

central vacuole A large, membrane-bounded sac found in plant cells that stores proteins, pigments, and waste materials.

centriole A cytoplasmic organelle identical in structure to a basal body; divides and organizes spindle fibers during mitosis and meiosis.

centromere A visible point of constriction on a chromosome that contains repeated DNA sequences that bind specific proteins.

cephalization The evolution of a head and brain area in the anterior end of animals.

cerebellum The hindbrain region of the vertebrate brain that integrates information about body position and motion.

cerebral cortex The thin surface layer of neurons and glial cells covering the cerebrum; the seat of conscious sensations.

cerebrum The portion of the vertebrate brain (the forebrain) that occupies the upper part of the skull; the primary association center.

chaetae Bristles of chitin on each body segment that help anchor annelid worms during locomotion.

channel protein (ion channel) A transmembrane protein with a hydrophilic interior that provides an aqueous channel allowing diffusion of specific ions.

chaperone protein A class of enzymes that help proteins fold into the correct configuration.

character displacement A process in which natural selection favors individuals in a species that use resources not used by other species.

character state In cladistics, one of two or more distinguishable forms of a character.

charging reaction The reaction by which an aminoacyl-tRNA synthetase attaches a specific amino acid to the correct tRNA using energy from ATP.

chelicera The first pair of appendages in horseshoe crabs, sea spiders, and arachnids—the chelicerates.

chemical synapse A close association that allows chemical communication between neurons via neurotransmitters.

chemiosmosis The mechanism by which ATP is generated in mitochondria and chloroplasts using proton pumps.

chiasma An X-shaped figure that can be seen in the light microscope during meiosis; evidence of crossing over.

chitin A tough, resistant, nitrogen-containing polysaccharide that forms the cell walls of certain fungi and arthropod exoskeletons.

chlorophyll The primary type of light-absorbing pigment in photosynthesis (Chlorophyll a and Chlorophyll b).

chloroplast A cell-like organelle present in algae and plants that contains chlorophyll and carries out photosynthesis.

choanocyte A specialized flagellated cell found in sponges; choanocytes line the body interior.

chorion The outer member of the double membrane that surrounds the embryo of reptiles, birds, and mammals.

chorionic villus sampling A technique in which fetal cells are sampled from the chorion of the placenta rather than from the amniotic fluid.

chromatid One of the two daughter strands of a duplicated chromosome that is joined by a single centromere.

chromatin The complex of DNA and proteins of which eukaryotic chromosomes are composed.

chromatin-remodeling complex A large protein complex that has been found to modify histones and DNA to change the structure of chromatin.

chromosomal mutation Any mutation that affects chromosome structure.

chromosomal theory of inheritance The theory stating that hereditary traits are carried on chromosomes.

chromosome The vehicle by which hereditary information is physically transmitted from one generation to the next.

cilium A short cellular projection from the surface of a eukaryotic cell, having the same internal structure as a flagellum.

circadian rhythm An endogenous cyclical rhythm that oscillates on a daily (24-hour) basis.

circulatory system A network of vessels in coelomate animals that carries fluids to and from different areas of the body.

cisterna A small collecting vessel that pinches off from the end of a Golgi body to form a transport vesicle.

cisternal space The inner region of a membrane-bounded structure; also called the lumen.

clade A taxonomic group composed of an ancestor and all its descendants.

cladistics A taxonomic technique used for creating hierarchies of organisms that represent true phylogenetic relationship.

cladogram A figure that depicts the evolutionary relationships among a group of species or other taxa.

class A taxonomic category between phyla and orders.

classical conditioning The repeated presentation of a stimulus in association with a response that causes the brain to form an association.

clathrin A protein located just inside the plasma membrane in eukaryotic cells, in indentations called clathrin-coated pits.

cleavage In vertebrates, a rapid series of successive cell divisions of a fertilized egg.

cleavage furrow The constriction that forms during cytokinesis in animal cells that is responsible for dividing the cell.

climax vegetation Vegetation encountered in a self-perpetuating community of plants that has stabilized.

cloaca In some animals, the common exit chamber from the digestive, reproductive, and urinary system.

clone-by-clone sequencing A method of genome sequencing in which a physical map is constructed first.

clonal selection Amplification of a clone of immune cells initiated by antigen recognition.

cloning Producing a cell line or culture all of whose members contain identical copies of a particular nucleotide sequence.

closed circulatory system A circulatory system in which the blood is physically separated from other body fluids.

coacervate A spherical aggregation of lipid molecules in water, held together by hydrophobic forces.

coactivator A protein that functions to link transcriptional activators to the transcription complex.

cochlea In terrestrial vertebrates, a tubular cavity of the inner ear containing the essential organs for hearing.

coding strand The strand of a DNA duplex that is the same as the RNA encoded by a gene.

codominance Describes a case in which two or more alleles of a gene are each dominant to other alleles but not to each other.

codon The basic unit of the genetic code; a sequence of three adjacent nucleotides in DNA or mRNA.

coelom In animals, a fluid-filled body cavity that develops entirely within the mesoderm.

coenzyme A nonprotein organic molecule such as NAD that plays an accessory role in enzyme-catalyzed processes.

coevolution The simultaneous development of adaptations in two or more populations that interact closely.

cofactor One or more nonprotein components required by enzymes in order to function.

cohesin A protein complex that holds sister chromatids together during cell division.

collecting duct The final portion of the renal tubule system where water can be selectively reabsorbed.

collenchyma cell In plants, the cells that form a supporting tissue; often found in regions of primary growth.

colloblast A specialized type of cell found in comb jellies that releases an adhesive substance to help capture prey.

colonial flagellate hypothesis The proposal that metazoans descended from colonial protists; supported by sponges.

commensalism A relationship in which one individual lives close to another and benefits, and the host is unaffected.

community All of the species inhabiting a common environment and interacting with one another.

companion cell A specialized parenchyma cell that is associated with each sieve-tube member in the phloem.

competitive exclusion The hypothesis that two species with identical requirements cannot exist in the same locality indefinitely.

competitive inhibitor An inhibitor that binds to the same active site as an enzyme’s substrate.

complementary Describes genetic information in which each nucleotide base has a complementary partner.

complementary DNA (cDNA) A DNA copy of an mRNA transcript; produced by the action of reverse transcriptase.

complement system The chemical defense of a vertebrate body that consists of a battery of proteins.

complete digestive system A digestive system that has both a mouth and an anus, allowing unidirectional flow.

compound eye An organ of sight in many arthropods composed of many independent visual units called ommatidia.

concentration gradient A difference in concentration of a substance from one location to another.

condensin A protein complex involved in condensation of chromosomes during mitosis and meiosis.

cone (1) In plants, the reproductive structure of a conifer. (2) In vertebrates, a type of light-sensitive neuron in the retina.

conidia An asexually produced fungal spore.

conjugation Temporary union of two unicellular organisms, during which genetic material is transferred.

consensus sequence In genome sequencing, the overall sequence that is consistent with the sequences of individual fragments.

conservation of synteny The preservation over evolutionary time of arrangements of DNA segments in related species.

contig A contiguous segment of DNA assembled by analyzing sequence overlaps from smaller fragments.

continuous variation Variation in a trait that occurs along a continuum, such as the trait of height in human beings.

contractile vacuole In protists and some animals, a clear fluid-filled vacuole that takes up water and releases it to the outside.

conus arteriosus The anteriormost chamber of the embryonic heart in vertebrate animals.

convergent evolution The independent development of similar structures in organisms that are not directly related.

copy number variation (CNV) DNA variation in a population that involves large stretches of DNA.

cork cambium The lateral meristem that forms the periderm, producing cork (phellem) toward the surface.

cornea The transparent outer layer of the vertebrate eye.

corolla The petals, collectively; usually the conspicuously colored flower whorl.

corpus callosum The band of nerve fibers that connects the two hemispheres of the cerebrum.

corpus luteum A structure that develops from a ruptured follicle in the ovary after ovulation.

cortex The outer layer of a structure; in plants, the primary ground tissue of a stem or root.

cotyledon A seed leaf that generally stores food in dicots or absorbs it in monocots.

crassulacean acid metabolism (CAM) A mode of carbon dioxide fixation by which CO2 enters leaf stomata at night.

CRISPR (Clustered Regular Interspersed Short Palindromic Repeats) Repeat elements found in prokaryotic DNA used as tools to edit genomes.

crista A folded extension of the inner membrane of a mitochondrion.

cross-current flow In bird lungs, the latticework of capillaries arranged across the air flow at a 90° angle.

crossing over In meiosis, the exchange of corresponding chromatid segments between homologous chromosomes.

ctenidia Respiratory gills of mollusks; rich in blood vessels.

cuticle A waxy or fatty, noncellular layer (formed of cutin) on the outer wall of epidermal cells.

cutin In plants, a fatty layer produced by the epidermis that forms the cuticle.

cyanobacteria A group of photosynthetic bacteria, sometimes called the “blue-green algae.”

cyclic AMP (cAMP) A form of adenosine monophosphate that acts as an intracellular second messenger.

cyclic photophosphorylation Reactions beginning with light absorption by chlorophyll that return excited electrons to the photosystem, generating ATP.

cyclin Proteins produced in synchrony with the cell cycle that combine with protein kinases.

cyclin-dependent kinase (Cdk) Protein kinase enzymes that control progress through the cell cycle.

cytochrome Iron-containing protein pigments that serve as electron carriers in transport chains.

cytochrome b6–f complex A proton pump found in the thylakoid membrane that uses energy to pump protons.

cytokine Signaling molecules secreted by immune cells that affect other immune cells.

cytokinesis Division of the cytoplasm of a cell after nuclear division.

cytoplasm The material within a cell, excluding the nucleus.

cytoskeleton A network of protein microfilaments and microtubules that maintains cell shape.

cytosol The fluid portion of the cytoplasm.

cytotoxic T cell A special T cell activated during cell-mediated immune response that destroys infected cells.

deamination The removal of an amino group from a compound.

deductive reasoning Logical application of general principles to predict results.

defensin Small cysteine-rich peptides with antimicrobial properties.

dehydration reaction Chemical reaction joining two molecules by removing water.

dehydrogenation Chemical reaction involving the loss of a hydrogen atom.

deletion Mutation in which a portion of a chromosome is lost.

demography Properties of the rate of growth and age structure of populations.

denaturation Loss of the native configuration of a protein or nucleic acid.

dendrite Branched process of a neuron conducting impulses toward the cell body.

dendritic cells Immune cells that act as antigen-presenting cells.

deoxyribonucleic acid (DNA) The genetic material of all organisms; a double helix.

dephosphorylation The removal of a phosphate group from a protein.

depolarization Movement of ions across a membrane wiping out potential difference.

derepression Production of enzymes when the end product of a pathway is absent.

derived character Characteristic representing a departure from the primitive form.

derived similarity Similarity that arose recently and is shared by a subset of species.

dermal tissue Tissue forming the outer layer of a multicellular body.

desmosome Anchoring junction connecting cytoskeletons with cadherin.

determinate development Development where each embryonic cell has a predetermined fate.

deuterostome Animals in which the anus develops first and the mouth second.

diacylglycerol (DAG) A second messenger released when phospholipase C cleaves PIP2.

diaphragm Muscle sheet separating thoracic and abdominal cavities.

diapsid Reptiles with two pairs of temporal openings in the skull.

diastolic pressure Minimum pressure between heartbeats.

dicer Enzyme that generates small RNA molecules by chopping up double-stranded RNA.

dichogamous Plant where male and female structures reach maturity at different times.

dicot Flowering plants with two cotyledons and net-veined leaves.

dideoxynucleotide Nucleotide used as a chain terminator in DNA sequencing.

differentiation Process where unspecialized cells become specialized.

diffusion Net movement of molecules from high to low concentration.

dihybrid An individual heterozygous at two different loci.

dihybrid cross Genetic cross involving two different traits.

dikaryotic Having pairs of nuclei within each cell.

dioecious Having male and female elements on different individuals.

diploid Having two sets of chromosomes (2n).

diplontic Life cycle strategy where only the diploid stage is multicellular.

directional selection Selection eliminating one extreme from a range of phenotypes.

disaccharide Carbohydrate formed of two simple sugar molecules.

disassortative mating Mating between phenotypically different individuals.

disruptive selection Selection favoring extreme types over the intermediate.

dissociation Reversible separation of protein subunits.

distal convoluted tubule Portion of the renal tubule feeding the collecting duct.

diurnal Active during the day.

DNA-binding motif Region of a regulatory protein capable of binding DNA.

DNA fingerprinting Technique to identify differences in the DNA of individuals.

DNA gyrase Topoisomerase involved in relieving torsional strain in DNA.

DNA library Collection of DNAs representing a complex mixture or genome.

DNA ligase Enzyme responsible for forming phosphodiester bonds.

DNA microarray Array of DNA fragments used to identify gene activity.

DNA polymerase Enzymes that synthesize DNA from a template.

domain Modular region of a protein or a high taxonomic level.

Domain Archaea Group containing highly diverse unicellular prokaryotes.

Domain Bacteria Group containing a vast group of unicellular prokaryotes.

Domain Eukarya Group containing eukaryotic organisms (protists, fungi, plants, animals).

dominant Allele expressed in both heterozygous and homozygous conditions.

dosage compensation Mechanism keeping gene expression same in both sexes.

double fertilization Fusion of egg and sperm resulting in a zygote and endosperm.

double helix Structure of DNA where two strands coil around an axis.

Down syndrome Genetic disease caused by trisomy 21.

duodenum Upper portion of the small intestine.

duplication Mutation in which a portion of a chromosome is duplicated.

ecdysis Shedding of outer, cuticular layer; molting, as in insects or crustaceans.

ecdysone Molting hormone of arthropods, which triggers when ecdysis occurs.

ecology The study of interactions of organisms with one another and with their physical environment.

ecosystem A major interacting system that includes organisms and their nonliving environment.

ecotype A locally adapted variant of an organism; differing genetically from other ecotypes.

ectoderm One of the three embryonic germ layers of early vertebrate embryos; ectoderm gives rise to the outer epithelium of the body (skin, hair, nails) and to the nerve tissue, including the sense organs, brain, and spinal cord.

ectomycorrhizae Externally developing mycorrhizae that do not penetrate the cells they surround.

ectotherms Animals such as reptiles, fish, or amphibians, whose body temperature is regulated by their behavior or by their surroundings.

electronegativity A property of atomic nuclei that refers to the affinity of the nuclei for valence electrons; a nucleus that is more electronegative has a greater pull on electrons than one that is less electronegative.

electron transport chain The passage of energetic electrons through a series of membrane-associated electron-carrier molecules to proton pumps embedded within mitochondrial or chloroplast membranes. See chemiosmosis.

elongation factor (EF-Tu) In protein synthesis in E. coli, a factor that binds to GTP and to a charged tRNA to accomplish binding of the charged tRNA to the A site of the ribosome, so that elongation of the polypeptide chain can occur.

embryo A multicellular developmental stage that follows cell division of the zygote.

embryonic stem cell (ES cell) A stem cell derived from an early embryo that can develop into different adult tissues and give rise to an adult organism when injected into a blastocyst.

emergent properties Novel properties arising from the way in which components interact. Emergent properties often cannot be deduced solely from knowledge of the individual components.

emerging virus Any virus that originates in one organism but then passes to another; usually refers to transmission to humans.

endergonic Describes a chemical reaction in which the products contain more energy than the reactants, so that free energy must be put into the reaction from an outside source to allow it to proceed.

endocrine gland Ductless gland that secretes hormones into the extracellular spaces, from which they diffuse into the circulatory system.

endocytosis The uptake of material into cells by inclusion within an invagination of the plasma membrane; the uptake of solid material is phagocytosis, and that of dissolved material is pinocytosis.

endoderm One of the three embryonic germ layers of early vertebrate embryos, destined to give rise to the epithelium that lines internal structures and most of the digestive and respiratory tracts.

endodermis In vascular plants, a layer of cells forming the innermost layer of the cortex in roots and some stems.

endomembrane system A system of connected membranous compartments found in eukaryotic cells.

endometrium The lining of the uterus in mammals; thickens in response to secretion of estrogens and progesterone and is sloughed off in menstruation.

endomycorrhizae Mycorrhizae that develop within cells.

endonuclease An enzyme capable of cleaving phosphodiester bonds between nucleotides located internally in a DNA strand.

endoplasmic reticulum (ER) Internal membrane system that forms a netlike array of channels and interconnections within the cytoplasm of eukaryotic cells. The ER is divided into rough (RER) and smooth (SER) compartments.

endorphin One of a group of small neuropeptides produced by the vertebrate brain; like morphine, endorphins modulate pain perception.

endosperm A storage tissue characteristic of the seeds of angiosperms, which develops from the union of a male nucleus and the polar nuclei of the embryo sac.

endospore A highly resistant, thick-walled bacterial spore that can survive harsh environmental stress, such as heat or dessication, and then germinate when conditions become favorable.

endosymbiosis Theory that proposes that eukaryotic cells evolved from a symbiosis between different species of prokaryotes.

endotherm An animal capable of maintaining a constant body temperature. See homeotherm.

energy level A discrete level, or quantum, of energy that an electron in an atom possesses. To change energy levels, an electron must absorb or release energy.

enhancer A site of regulatory protein binding on the DNA molecule distant from the promoter and start site for a gene’s transcription.

enthalpy In a chemical reaction, the energy contained in the chemical bonds of the molecule, symbolized as H; in a cellular reaction, the free energy is equal to the enthalpy of the reactant molecules in the reaction.

entropy A measure of the randomness or disorder of a system; a measure of how much energy in a system has become so dispersed that it is no longer available to do work.

enzyme A protein that is capable of speeding up specific chemical reactions by lowering the required activation energy.

enzyme–substrate complex The complex formed when an enzyme binds with its substrate. This complex often has an altered configuration compared with the nonbound enzyme.

ependymal cells Neuroglial cells found in the central nervous system that produce cerebrospinal fluid.

epicotyl The region just above where the cotyledons are attached.

epidermal cell In plants, a cell that collectively forms the outermost layer of the primary plant body; includes specialized cells such as trichomes and guard cells.

epidermis The outermost layers of cells; in plants, the exterior primary tissue of leaves, young stems, and roots; in vertebrates, the nonvascular external layer of skin.

epididymis A sperm storage vessel; a coiled part of the sperm duct that lies near the testis.

epigenetic inheritance Cell-to-cell and organism-to-organism inheritance of a trait that does not involve alterations to DNA.

epistasis Interaction between two nonallelic genes in which one of them modifies the phenotypic expression of the other.

epithelium In animals, a type of tissue that covers an exposed surface or lines a tube or cavity.

equilibrium A stable condition; the point at which a chemical reaction proceeds as rapidly in the reverse direction as it does in the forward direction.

erythrocyte Red blood cell, the carrier of hemoglobin.

erythropoiesis The manufacture of blood cells in the bone marrow.

E site In a ribosome, the exit site that binds to the tRNA that carried the previous amino acid added to the polypeptide chain.

estrus The period of maximum female sexual receptivity, associated with ovulation of the egg.

ethology The study of patterns of animal behavior in nature.

euchromatin That portion of a eukaryotic chromosome that is transcribed into mRNA; contains active genes that are not tightly condensed during interphase.

eukaryote A cell characterized by membrane-bounded organelles, most notably the nucleus, and one that possesses chromosomes whose DNA is associated with proteins.

eutherian A placental mammal.

eutrophic Refers to a lake in which an abundant supply of minerals and organic matter exists.

evolution Genetic change in a population of organisms; in general, evolution leads to progressive change from simple to complex.

excision repair A nonspecific mechanism to repair damage to DNA during synthesis. The damaged or mismatched region is excised, and DNA polymerase replaces the region removed.

exergonic Describes a chemical reaction in which the products contain less free energy than the reactants, so that free energy is released in the reaction.

exhalant siphon In bivalve mollusks, the siphon through which outgoing water leaves the body.

exocrine gland A type of gland that releases its secretion through a duct, such as a digestive gland or a sweat gland.

exocytosis A type of bulk transport out of cells in which a vacuole fuses with the plasma membrane, discharging the vacuole’s contents to the outside.

exon A segment of DNA that is both transcribed into RNA and translated into protein. See intron.

exonuclease An enzyme capable of cutting phosphodiester bonds between nucleotides located at an end of a DNA strand.

exoskeleton An external skeleton, as in arthropods.

experiment A test of one or more hypotheses based on contrasting predictions that can be tested experimentally in control and test experiments.

expiratory reserve volume The amount of air that can be exhaled above and beyond the tidal volume.

expressed sequence tag (EST) A short sequence of a cDNA that unambiguously identifies the cDNA.

expression vector A type of vector (plasmid or phage) that contains the sequences necessary to drive expression of inserted DNA in a specific cell type.

external respiration The exchange of gases between an organism and the environment.

exteroceptor A receptor that is excited by stimuli from the external world.

extremophile An archaeal organism that lives in extreme environments, such as hot springs, highly saline environments, or under high pressure at the bottom of oceans.

5′ cap In eukaryotes, a structure added to the 5′ end of an mRNA consisting of methylated GTP attached by a 5′ to 5′ bond. The cap protects this end from degradation and is involved in the initiation of translation.

F+ A bacterial cell that harbors a fertility plasmid.

facilitated diffusion The diffusion of molecules or ions through carrier proteins or ion channels. Requires a concentration gradient, but not any input of energy.

family A taxonomic grouping of similar species above the level of genus.

fat A molecule composed of glycerol and three fatty acid molecules.

feedback inhibition Control mechanism whereby an increase in the concentration of some molecules inhibits the synthesis of that molecule.

fermentation The enzyme-catalyzed extraction of energy from organic compounds without the involvement of oxygen.

fertilization The fusion of two haploid gamete nuclei to form a diploid zygote nucleus.

fibroblast A flat, irregularly branching cell of connective tissue that secretes structurally strong proteins into the matrix between the cells.

first filial (F1) generation The offspring resulting from a cross between a parental generation (P); in experimental crosses, these parents usually have different phenotypes.

First Law of Thermodynamics Energy cannot be created or destroyed, but can only undergo conversion from one form to another; thus, the amount of energy in the universe is unchangeable.

fitness The genetic contribution of an individual to succeeding generations. Relative fitness refers to the fitness of an individual relative to other individuals in a population.

fixed action pattern A stereotyped animal behavior response, thought by ethologists to be based on programmed neural circuits.

flagellin The protein composing bacterial flagella, which allow a cell to move through an aqueous environment.

flagellum A long, threadlike structure protruding from the surface of a cell and used in locomotion.

flame cell A specialized cell found in the network of tubules inside flatworms that assists in water regulation and some waste excretion.

flavin adenine dinucleotide (FAD, FADH2) A cofactor that acts as a soluble (not membrane bound) electron carrier (can be reversibly oxidized and reduced).

fluorescent in situ hybridization (FISH) A cytological method used to find specific DNA sequences on chromosomes with a specific fluorescently labeled probe.

food security Having access to sufficient, safe food to avoid malnutrition and starvation; a global human issue.

foraging behavior A collective term for the many complex, evolved behaviors that influence what an animal eats and how the food is obtained.

founder effect The effect by which rare alleles and combinations of alleles may be enhanced in new populations.

fovea A small depression in the center of the retina with a high concentration of cones; the area of sharpest vision.

F plasmids Circular pieces of DNA that confer the ability to produce a sex pilus in certain bacterial species.

frameshift mutation A mutation in which a base is added or deleted from the DNA sequence. These changes alter the reading frame downstream of the mutation.

free energy Energy available to do work.

free radical An ionized atom with one or more unpaired electrons, resulting from electrons that have been energized by ionizing radiation being ejected from the atom; free radicals react violently with other molecules, such as DNA, causing damage by mutation.

frequency-dependent selection A type of selection that depends on how frequently or infrequently a phenotype occurs in a population.

fruit In angiosperms, a mature, ripened ovary (or group of ovaries), containing the seeds.

functional genomics The study of the function of genes and their products, beyond simply ascertaining gene sequences.

functional group A molecular group attached to a hydrocarbon that confers chemical properties or reactivities. Examples include hydroxyl (–OH), carboxylic acid (–COOH) and amino groups (–NH2).

fundamental niche Also referred to as the hypothetical niche, this is the entire niche an organism could fill if there were no other interacting factors (such as competition or predation).

G0 phase The stage of the cell cycle occupied by cells that are not actively dividing.

G1 phase The phase of the cell cycle after cytokinesis and before DNA replication called the first “gap” phase. This phase is the primary growth phase of a cell.

G1/S checkpoint The primary control point at which a cell “decides” whether or not to divide. Also called START and the restriction point.

G2 phase The phase of the cell cycle between DNA replication and mitosis called the second “gap” phase. During this phase, the cell prepares for mitosis.

G2/M checkpoint The second cell-division control point, at which division can be delayed if DNA has not been properly replicated or is damaged.

gametangium A cell or organ in which gametes are formed.

gamete A haploid reproductive cell.

gametocytes Cells in the malarial sporozoite life cycle capable of giving rise to gametes when in the correct host.

gametophyte In plants, the haploid (n), gamete producing generation, which alternates with the diploid (2n) sporophyte.

ganglion An aggregation of nerve cell bodies; in invertebrates, ganglia are the integrative centers; in vertebrates, the term is restricted to aggregations of nerve cell bodies located outside the central nervous system.

gap gene Any of certain genes in Drosophila development that divide the embryo into large blocks in the process of segmentation; hunchback is a gap gene.

gap junction A junction between adjacent animal cells that allows the passage of materials between the cells.

gastrodermis In eumetazoan animals, the layer of digestive tissue that develops from the endoderm.

gastrula In vertebrates, the embryonic stage in which the blastula with its single layer of cells turns into a three-layered embryo made up of ectoderm, mesoderm, and endoderm.

gastrulation Developmental process that converts blastula into embryo with three embryonic germ layers: endoderm, mesoderm, and ectoderm. Involves massive cell migration to convert the hollow structure into a three-layered structure.

gene The basic unit of heredity; a sequence of DNA nucleotides on a chromosome that encodes a protein, tRNA, or rRNA molecule, or regulates the transcription of such a sequence.

gene conversion Alteration of one homologous chromosome by the cell’s error-detection and repair system to make it resemble the sequence on the other homologue.

gene expression The conversion of the genotype into the phenotype; the process by which DNA is transcribed into RNA, which is then translated into a protein product.

gene flow The movement of alleles from one population to another.

gene pool All the alleles present in a species.

gene-for-gene hypothesis A plant defense mechanism in which a specific protein encoded by a viral, bacterial, or fungal pathogen binds to a protein encoded by a plant gene and triggers a defense response in the plant.

general transcription factor Any of a group of transcription factors that are required for formation of an initiation complex by RNA polymerase II at a promoter. This allows a basal level that can be increased by the action of specific factors.

generalized transduction A form of gene transfer in prokaryotes in which any gene can be transferred between cells. This uses a lytic bacteriophage as a carrier where the virion is accidentally packaged with host DNA.

genetic counseling The process of evaluating the risk of genetic defects occurring in offspring, testing for these defects in unborn children, and providing the parents with information about these risks and conditions.

genetic drift Random fluctuation in allele frequencies over time by chance.

genetic map An abstract map that places the relative location of genes on a chromosome based on recombination frequency.

genetic marker Any genetic difference that can be detected and used to discriminate between individuals.

genome The entire DNA sequence of an organism.

genomic imprinting Describes an exception to Mendelian genetics in some mammals in which the phenotype caused by an allele is exhibited when the allele comes from one parent, but not from the other.

genomic library A DNA library that contains a representation of the entire genome of an organism.

genomics The study of genomes as opposed to individual genes.

genotype The genetic constitution underlying a single trait or set of traits.

genotype frequency A measure of the occurrence of a genotype in a population, expressed as a proportion of the entire population.

genus A taxonomic group that ranks below a family and above a species.

germination The resumption of growth and development by a spore or seed.

germ layers The three cell layers formed at gastrulation of the embryo that foreshadow the future organization of tissues; the layers, from the outside inward, are the ectoderm, the mesoderm, and the endoderm.

germ-line cells During zygote development, cells that are set aside from the somatic cells and that will eventually undergo meiosis to produce gametes.

gill (1) In aquatic animals, a respiratory organ, usually a thin-walled projection from some part of the external body surface, endowed with a rich capillary bed and having a large surface area. (2) In basidiomycete fungi, the plates on the underside of the cap.

globular protein Proteins with a compact tertiary structure with hydrophobic amino acids mainly in the interior.

glomerular filtrate The fluid that passes out of the capillaries of each glomerulus.

glomerulus A cluster of capillaries enclosed by Bowman’s capsule.

glucagon A vertebrate hormone produced in the pancreas that acts to initiate the breakdown of glycogen to glucose subunits.

gluconeogenesis The synthesis of glucose from noncarbohydrates (such as proteins or fats).

glucose A common six-carbon sugar (C6H12O6); the most common monosaccharide in most organisms.

glucose repression In E. coli, the preferential use of glucose even when other sugars are present; transcription of mRNA encoding the enzymes for utilizing the other sugars does not occur.

glyceraldehyde-3-phosphate (G3P) An intermediate in glycolysis that is a product of the cleavage reaction of fructose 1,6-bisphosphate.

glycocalyx A “sugar coating” on the surface of a cell resulting from the presence of polysaccharides on glycolipids and glycoproteins embedded in the outer layer of the plasma membrane.

glycogen Animal starch; a complex branched polysaccharide that serves as a food reserve in animals, bacteria, and fungi.

glycolipid Lipid molecule modified within the Golgi complex by having a short sugar chain (polysaccharide) attached.

glycolysis The anaerobic breakdown of glucose; this enzyme-catalyzed process yields two molecules of pyruvate with a net of two molecules of ATP.

glycoprotein Protein molecule modified within the Golgi complex by having a short sugar chain (polysaccharide) attached.

glyoxysome A small cellular organelle or microbody containing enzymes necessary for conversion of fats into carbohydrates.

glyphosate A biodegradable herbicide that works by inhibiting EPSP synthetase, a plant enzyme that makes aromatic amino acids.

Golgi apparatus (Golgi body) A collection of flattened stacks of membranes in the cytoplasm of eukaryotic cells; functions in collection, packaging, and distribution of molecules synthesized in the cell.

Golgi tendon organ Sensory organ found in skeletal muscles that monitors the stress on tendons.

G protein A protein that binds guanosine triphosphate (GTP) and assists in the function of cell-surface receptors. When the receptor binds its signal molecule, the G protein binds GTP and is activated to start a chain of events within the cell.

G protein-coupled receptor (GPCR) A receptor that acts through a heterotrimeric (three component) G protein to activate effector proteins. The effector proteins then function as enzymes to produce second messengers such as cAMP or IP3.

gradualism The view that species change very slowly in ways that may be imperceptible from one generation to the next but that accumulate and lead to major changes over thousands or millions of years.

Gram stain Staining technique that divides bacteria into gram-negative or gram-positive based on retention of a violet dye.

granum A stacked column of flattened, interconnected disks (thylakoids) that are part of the thylakoid membrane system in chloroplasts.

gravitropism Growth response to gravity in plants; formerly called geotropism.

ground meristem The primary meristem, or meristematic tissue, that gives rise to the plant body (except for the epidermis and vascular tissues).

ground tissue In plants, a type of tissue that performs many functions, including support, storage, secretion, and photosynthesis; may consist of many cell types.

growth factor Any of a number of proteins that bind to membrane receptors and initiate intracellular signaling systems that result in cell growth and division.

guard cell In plants, one of a pair of sausage-shaped cells flanking a stoma; the guard cells open and close the stomata.

guttation The exudation of liquid water from leaves due to root pressure.

gymnosperm A seed plant with seeds not enclosed in an ovary; conifers are gymnosperms.

gynoecium The aggregate of carpels in the flower of a seed plant.

habitat The environment of an organism; the place where it is usually found.

habituation A form of learning; a diminishing response to a repeated stimulus.

halophyte A plant that is salt-tolerant.

handicap hypothesis The hypothesis that males will evolve exaggerated traits that are disadvantageous and that by choosing a male with the largest handicap, females ensure that their offspring will receive these quality genes.

haplodiploidy A phenomenon occurring in certain organisms such as wasps, wherein both haploid (male) and diploid (female) individuals are encountered.

haplodiplontic A life cycle strategy in which both haploid and diploid stages are multicellular.

haploid Having only one set of chromosomes (n), in contrast to diploid (2n).

haplotype A region of a chromosome that is usually inherited intact, that is, it does not undergo recombination. These are identified based on analysis of SNPs.

Hardy-Weinberg equilibrium A mathematical description of the fact that allele and genotype frequencies remain constant in a random-mating population in the absence of inbreeding, selection, or other evolutionary forces.

Haversian canal Narrow channels that run parallel to the length of a bone and contain blood vessels and nerve cells.

heat A measure of the random motion of molecules; the greater the heat, the greater the motion. Heat is one form of kinetic energy.

heat of vaporization The amount of energy required to change 1 g of a substance from a liquid to a gas.

heavy metal Any of the metallic elements with high atomic numbers, such as arsenic, cadmium, lead, etc. Many heavy metals are toxic to animals even in small amounts.

helicase Any of a group of enzymes that unwind the two DNA strands in the double helix to facilitate DNA replication.

helix-turn-helix motif A common DNA-binding motif found in regulatory proteins; it consists of two α-helices linked by a nonhelical segment (the “turn”).

helper T cell A class of white blood cells that initiates both the cell-mediated immune response and the humoral immune response; helper T cells are the targets of the AIDS virus (HIV).

hemoglobin A globular protein in vertebrate red blood cells and in the plasma of many invertebrates that carries oxygen and carbon dioxide.

hemophilia An X-linked recessive genetic disorder that affects proteins involved in blood clotting. Symptoms include excessive bleeding due to failure of proper blood clotting.

hematopoietic stem cell The cells in bone marrow where blood cells are formed.

hermaphroditism Condition in which an organism has both male and female functional reproductive organs.

heterochromatin The portion of a eukaryotic chromosome that is not transcribed into RNA; remains condensed in interphase and stains intensely in histological preparations.

heterochrony An alteration in the timing of developmental events due to a genetic change; for example, a mutation that delays flowering in plants.

heterokaryotic In fungi, having two or more genetically distinct types of nuclei within the same mycelium.

heterosporous In vascular plants, having spores of two kinds, namely, microspores and megaspores.

heterotroph An organism that cannot derive energy from photosynthesis or inorganic chemicals, and so must feed on other plants and animals, obtaining chemical energy by degrading their organic molecules.

heterozygote advantage The situation in which individuals heterozygous for a trait have a selective advantage over those who are homozygous; an example is sickle cell anemia.

heterozygous Having two different alleles of the same gene; the term is usually applied to one or more specific loci, as in “heterozygous with respect to the W locus” (that is, the genotype is W/w).

Hfr cell An E. coli cell that has a high frequency of recombination due to integration of an F plasmid into its genome.

histone protein Any of eight proteins with an overall positive charge that associate in a complex. The DNA duplex coils around a core of eight histone proteins, held by its negatively charged phosphate groups, forming a nucleosome.

holoblastic cleavage Process in vertebrate embryos in which the cleavage divisions all occur at the same rate, yielding a uniform cell size in the blastula.

homeobox A sequence of 180 nucleotides located in homeotic genes that produces a 60-amino-acid peptide sequence (the homeodomain) active in transcription factors.

homeosis A change in the normal spatial pattern of gene expression that can result in homeotic mutants where a wild-type structure develops in the wrong place in or on the organism.

homeostasis The maintenance of a relatively stable internal physiological environment in an organism; usually involves some form of feedback self-regulation.

homeotherm An organism, such as a bird or mammal, capable of maintaining a stable body temperature independent of the environmental temperature.

homeotic gene One of a series of “master switch” genes that determine the form of segments developing in the embryo.

hominid Any primate in the human family, Hominidae. Homo sapiens is the only living representative.

hominoid Collectively, hominids and apes; the monkeys and hominoids constitute the anthropoid primates.

homokaryotic In fungi, having nuclei with the same genetic makeup within a mycelium.

homologous (1) Refers to similar structures that have the same evolutionary origin. (2) Refers to a pair of the same kind of chromosome in a diploid cell.

homologue One of a pair of chromosomes of the same kind located in a diploid cell; one copy of each pair of homologues comes from each gamete that formed the zygote.

homoplasy In cladistics, a shared character state that has not been inherited from a common ancestor exhibiting that state; may result from convergent evolution or evolutionary reversal.

homosporous In some plants, production of only one type of spore rather than differentiated types.

homozygous Being a homozygote, having two identical alleles of the same gene; the term is usually applied to one or more specific loci (e.g., W/W or w/w).

horizontal gene transfer (HGT) The passing of genes laterally between species; more prevalent very early in the history of life.

hormone A molecule, usually a peptide or steroid, that is produced in one part of an organism and triggers a specific cellular reaction in target tissues and organs some distance away.

host range The range of organisms that can be infected by a particular virus.

Hox gene A group of homeobox-containing genes that control developmental events, usually found organized into clusters of genes. These genes have been conserved in many different multicellular animals.

humoral immunity Arm of the adaptive immune system involving B cells that produce soluble antibodies specific for foreign antigens.

humus Partly decayed organic material found in topsoil.

hybridization The mating of unlike parents.

hydration shell A “cloud” of water molecules surrounding a dissolved substance, such as sucrose or Na+ and Cl– ions.

hydrogen bond A weak association formed with hydrogen in polar covalent bonds. In water, oxygen and hydrogen in different water molecules form hydrogen bonds.

hydrolysis reaction A reaction that breaks a bond by the addition of water. This is the reverse of dehydration.

hydrophilic Literally translates as “water-loving” and describes substances that are soluble in water. These must be either polar or charged (ions).

hydrophobic Literally translates as “water-fearing” and describes nonpolar substances that are not soluble in water.

hydrophobic exclusion The tendency of nonpolar molecules to aggregate together when placed in water. Exclusion refers to the action of water in forcing these molecules together.

hydrostatic skeleton The skeleton of most soft-bodied invertebrates that have neither an internal nor an external skeleton. They use the relative incompressibility of the water within their bodies as a kind of skeleton.

hyperpolarization Above-normal negativity of a cell membrane during its resting potential.

hypersensitive response Plants respond to pathogens by selectively killing plant cells to block the spread of the pathogen.

hypertonic A solution with a higher concentration of solutes than the cell. A cell in a hypertonic solution tends to lose water by osmosis.

hypha A filament of a fungus or oomycete; collectively, the hyphae constitute the mycelium.

hypocotyl The region immediately below where the cotyledons are attached.

hypothalamus A region of the vertebrate brain just below the cerebral hemispheres, under the thalamus; a center of the autonomic nervous system.

hypotonic A solution with a lower concentration of solutes than the cell. A cell in a hypotonic solution tends to take in water by osmosis.

icosahedron A structure consisting of 20 equilateral triangular facets; this is commonly seen in viruses and forms one kind of viral capsid.

imaginal disk One of about a dozen groups of cells set aside in the abdomen of a larval insect and committed to forming key parts of the adult insect’s body.

immune response In vertebrates, a defensive reaction of the body to invasion by a foreign substance or organism. See antibody and B cell.

immunoglobulin An antibody molecule.

immunological tolerance Process where the immune system learns to not react to self-antigens.

inbreeding The breeding of genetically related plants or animals; inbreeding tends to increase homozygosity.

inclusive fitness Describes the sum of the number of genes directly passed on in an individual’s offspring and those genes passed on indirectly by kin (other than offspring) whose existence results from the benefit of the individual’s altruism.

incomplete dominance Describes a case in which two or more alleles of a gene do not display clear dominance. The phenotype of a heterozygote is intermediate between the homozygous forms.

independent assortment In a dihybrid cross, describes the random assortment of alleles for each of the genes. For genes on different chromosomes this results from the random orientations of different homologous pairs during metaphase I of meiosis.

indeterminate development A type of development in animals in which the first few embryonic cells are identical daughter cells, any one of which could develop separately into a complete organism; their fate is indeterminate.

indoleacetic acid (IAA) A member of the auxin family of plant hormones that elicits developmental changes in response to environmental cues such as light.

indolebutyric acid (IBA) A synthetic auxin used to control fruit formation and ripening in agriculture.

inducer exclusion Part of the mechanism of glucose repression in E. coli in which the presence of glucose prevents the entry of lactose such that the lac operon cannot be induced.

induction (1) Production of enzymes in response to a substrate; a mechanism by which binding of an inducer to a repressor allows transcription of an operon. (2) In embryonic development, the process by which the development of a cell is influenced by interaction with an adjacent cell.

inductive reasoning The logical application of specific observations to make a generalization. In science, inductive reasoning is used to formulate testable hypotheses.

industrial melanism Phrase used to describe the evolutionary process in which initially light-colored organisms become dark as a result of natural selection.

inflammatory response A generalized nonspecific response to infection that acts to clear an infected area of infecting microbes and dead tissue cells so that tissue repair can begin.

inhalant siphon In bivalve mollusks, the siphon through which incoming water enters the body.

inheritance of acquired characteristics Also known as Lamarckism; the theory, now discounted, that individuals genetically pass on to their offspring physical and behavioral changes developed during the individuals’ own lifetime.

inhibitor A substance that binds to an enzyme and decreases its activity.

initiation factor One of several proteins involved in the formation of an initiation complex in prokaryote polypeptide synthesis.

initiator tRNA A tRNA molecule involved in the beginning of translation. In prokaryotes, the initiator tRNA is charged with N-formylmethionine (tRNAfMet).

inorganic phosphate A phosphate molecule that is not a part of an organic molecule; added and removed in the formation and breakdown of ATP.

inositol-1,4,5-trisphosphate (IP3) Second messenger produced by the cleavage of phosphatidylinositol-4,5-bisphosphate.

insertional inactivation Destruction of a gene’s function by the insertion of a transposon.

inspiratory reserve volume The amount of air that can be inhaled above and beyond the tidal volume.

instar A larval developmental stage in insects.

integrin Any of a group of cell-surface proteins involved in adhesion of cells to substrates. Critical to migrating cells moving through the cell matrix.

intercalary meristem A type of meristem that arises in stem internodes in some plants, such as corn and horsetails; responsible for elongation of the internodes.

interferon In vertebrates, a protein produced in virus-infected cells that inhibits viral multiplication.

intermediate filaments A cytoskeletal element made up of fibrous lamin proteins that is intermediate in size between microfilaments and microtubules.

intermembrane space The outer compartment of a mitochondrion that lies between the two membranes.

internal respiration The transport of gases within an organism.

interneuron (association neuron) A nerve cell found only in the middle of the spinal cord that acts as a functional link between sensory neurons and motor neurons.

internode In plants, the region of a stem between two successive nodes.

interoceptor A receptor that senses information related to the body itself, its internal condition, and its position.

interphase The period between two mitotic or meiotic divisions in which a cell grows and its DNA replicates.

intersexual selection Selection by members of one sex of which individuals in the other sex will get to mate.

intrasexual selection Competitive interactions among members of one sex to achieve fertilization success.

intron Portion of mRNA as transcribed from eukaryotic DNA that is removed by enzymes before the mature mRNA is translated into protein.

inversion A reversal in order of a segment of a chromosome; also, to turn inside out, as in embryogenesis of sponges.

in vitro mutagenesis The ability to create mutations at any site in a cloned gene to examine the mutations’ effects on function.

ion channel Membrane proteins that allow the passage of ions through a phospholipid bilayer. These are usually specific for a single ion.

ionizing radiation High-energy radiation that is highly mutagenic, producing free radicals that react with DNA; includes X-rays and γ-rays.

isomer One of a group of molecules identical in atomic composition but differing in structural arrangement.

isotonic A solution having the same concentration of solutes as the cell. A cell in an isotonic solution takes in and loses the same amount of water.

isotope Different forms of the same element with the same number of protons but different numbers of neutrons.

jasmonic acid An organic molecule that is part of a plant’s wound response; it signals the production of a proteinase inhibitor.

karyotype The morphology of the chromosomes of an organism as viewed with a light microscope.

keratin A tough, fibrous protein formed in epidermal tissues and modified into skin, feathers, hair, and hard structures such as horns and nails.

key innovation A newly evolved trait in a species that allows members to use resources or other aspects of the environment that were previously inaccessible.

kidney In vertebrates, the organ that filters the blood to remove nitrogenous wastes and regulates the balance of water and solutes in blood plasma.

kilocalorie Unit describing the amount of heat required to raise the temperature of a kilogram of water by 1°C; sometimes called a Calorie.

kinase cascade A series of protein kinases that phosphorylate each other in succession; can amplify signals during signal transduction.

kinesis Changes in activity level in an animal that are dependent on stimulus intensity. See kinetic energy.

kinetic energy The energy of motion.

kinetochore Disk-shaped protein structure within the centromere to which the spindle fibers attach during mitosis or meiosis.

kingdom The second-highest commonly used taxonomic category.

kin selection Selection favoring relatives; an increase in the frequency of related individuals in a population, leading to an increase in those alleles shared by the group.

knockout mice Mice in which a known gene is inactivated (“knocked out”) using recombinant DNA and ES cells.

Krebs cycle Another name for the citric acid cycle; also called the tricarboxylic acid (TCA) cycle.

labrum The upper lip of insects and crustaceans situated above or in front of the mandibles.

lac operon In E. coli, the operon containing genes that encode the enzymes to metabolize lactose.

lagging strand The DNA strand that must be synthesized discontinuously because of the 5′-to-3′ directionality of DNA polymerase during replication and the antiparallel nature of DNA.

larva A developmental stage that is unlike the adult found in organisms that undergo metamorphosis.

larynx The voice box; a cartilaginous organ that lies between the pharynx and trachea and is responsible for sound production in vertebrates.

lateral line system A sensory system encountered in fish, through which mechanoreceptors in a line down the side of the fish are sensitive to motion.

lateral meristems In vascular plants, the meristems that give rise to secondary tissue; the vascular cambium and cork cambium.

Law of Independent Assortment Mendel’s second law of heredity, stating that genes located on nonhomologous chromosomes assort independently of one another.

Law of Segregation Mendel’s first law of heredity, stating that alternative alleles for the same gene segregate from each other in production of gametes.

leading strand The DNA strand that can be synthesized continuously from the origin of replication.

leaf primordium A lateral outgrowth from the apical meristem that will eventually become a leaf.

lenticels Spongy areas in the cork surfaces of stem, roots, and other plant parts that allow interchange of gases between internal tissues and the atmosphere.

leucine zipper motif A motif in regulatory proteins in which two different protein subunits associate to form a single DNA-binding site; the proteins are connected by leucines.

leucoplast In plant cells, a colorless plastid in which starch grains are stored; usually found in cells not exposed to light.

leukocyte A white blood cell; a diverse array of nonhemoglobin-containing blood cells, including phagocytic macrophages and antibody-producing lymphocytes.

lichen Symbiotic association between a fungus and a photosynthetic organism such as a green alga or cyanobacterium.

ligand A signaling molecule that binds to a specific receptor protein, initiating signal transduction in cells.

light-dependent reactions In photosynthesis, the reactions in which light energy is captured and used in production of ATP and NADPH.

light-independent reactions In photosynthesis, the reactions of the Calvin cycle in which ATP and NADPH are used to reduce CO2 and produce organic compounds such as glucose.

lignin A highly branched polymer that makes plant cell walls more rigid; an important component of wood.

limbic system The hypothalamus, together with the network of neurons that link it to areas of the cerebral cortex; responsible for deep-seated drives and emotions.

linked genes Genes that are physically close together and therefore tend to segregate together.

linkage disequilibrium Association of alleles for 2 or more loci in a population that is higher than expected by chance.

lipase An enzyme that catalyzes the hydrolysis of fats.

lipid A nonpolar hydrophobic organic molecule that is insoluble in water but dissolves readily in nonpolar organic solvents.

lipid bilayer The structure of a cellular membrane, in which two layers of phospholipids spontaneously align.

lipid droplet An organelle used by cells to store lipids.

lipopolysaccharide A lipid with a polysaccharide molecule attached; found in the outer membrane layer of gram-negative bacteria.

locus The position on a chromosome where a gene is located.

long interspersed element (LINE) Any of a type of large transposable element found in humans and other primates.

long terminal repeat (LTR) A particular type of retrotransposon that has repeated elements at its ends.

loop of Henle In the kidney, a hairpin-shaped portion of the renal tubule in which water and salt are reabsorbed.

lophophore A horseshoe-shaped crown of ciliated tentacles that surrounds the mouth of certain spiralian animals.

lumen A term for any bounded opening, such as the cisternal space of the endoplasmic reticulum.

luteal phase The second phase of the female reproductive cycle, during which the mature eggs are released into the Fallopian tubes.

lymph In animals, a colorless fluid derived from blood by filtration through capillary walls in the tissues.

lymphatic system In animals, an open vascular system that reclaims water that has entered interstitial regions from the bloodstream.

lymphocyte A type of white blood cell responsible for the immune response; principal classes are B cells and T cells.

lymphokine A regulatory molecule that is secreted by lymphocytes.

lysis Disintegration of a cell by rupture of its plasma membrane.

lysogenic cycle A viral cycle in which the viral DNA becomes integrated into the host chromosome and is replicated during cell reproduction.

lysosome A membrane-bounded vesicle containing digestive enzymes that is produced by the Golgi apparatus.

lytic cycle A viral cycle in which the host cell is killed (lysed) by the virus after viral duplication.

macroevolution The creation of new species and the extinction of old ones.

macromolecule An extremely large biological molecule; refers specifically to proteins, nucleic acids, polysaccharides, and lipids.

macronucleus A diploid nucleus found in ciliates in which genes are transcribed to produce the required proteome.

macronutrients Inorganic chemical elements required in large amounts for plant growth, such as nitrogen, potassium, and calcium.

macrophage A large phagocytic cell that is able to engulf and digest cellular debris and invading bacteria.

madreporite A sievelike plate on the surface of echinoderms through which water enters the water–vascular system.

MADS box gene Any of a family of genes that are the predominant homeotic genes of plants.

major groove The larger of the two grooves in a DNA helix, where regulatory proteins can recognize and bind to specific sequences.

major histocompatibility complex (MHC) A set of protein cell-surface markers that the immune system uses to identify “self.”

Malpighian tubules Blind tubules opening into the hindgut of terrestrial arthropods that function as excretory organs.

mandibles In crustaceans, insects, and myriapods, the appendages immediately posterior to the antennae used to seize or bite food.

mantle The soft, outermost layer of the body wall in mollusks; the mantle secretes the shell.

map unit Each 1% of recombination frequency between two genetic loci; also termed a centimorgan (cM).

marsupial A mammal in which the young are born early in their development and are typically retained in a pouch.

mass extinction A relatively sudden, sharp decline in the number of species, such as at the end of the Cretaceous period.

mass flow hypothesis The overall process by which materials move in the phloem of plants.

mast cells Leukocytes with granules containing molecules that initiate inflammation.

maternal inheritance A mode of uniparental inheritance from the female parent; in humans, mitochondria are inherited this way.

matrix In mitochondria, the solution in the interior space that contains the enzymes involved in oxidative respiration.

medusa A free-floating, often umbrella-shaped body form found in cnidarian animals, such as jellyfish.

megapascal (MPa) A unit of measure used for pressure in water potential.

megaphyll In plants, a leaf that has several to many veins connecting it to the vascular cylinder of the stem.

meiosis I The first round of cell division in meiosis, referred to as a “reduction division” where homologous chromosomes separate.

meiosis II The second round of division in meiosis, during which sister chromatids separate to produce four haploid daughter cells.

melatonin A hormone secreted by the pineal gland that is synthesized from the amino acid tryptophan.

membrane receptor A signal receptor present as an integral protein in the cell membrane, such as GPCRs or RTKs.

Mendelian ratio The characteristic dominant-to-recessive phenotypic ratios that Mendel observed (e.g., 3:1 or 9:3:3:1).

menstruation Periodic sloughing off of the blood-enriched lining of the uterus when pregnancy does not occur.

meristem Undifferentiated plant tissue from which new cells arise.

meroblastic cleavage A type of cleavage in the eggs of reptiles, birds, and some fish that occurs only on the blastodisc.

mesenchymal stem cell A type of stem cell that gives rise to connective tissues.

mesoderm One of the three embryonic germ layers; gives rise to muscle, bone, and the circulatory system.

mesoglea A layer of gelatinous material found between the epidermis and gastrodermis of eumetazoans.

mesohyl A gelatinous, protein-rich matrix found between the choanocyte layer and the epithelial layer of a sponge.

mesophyll The photosynthetic parenchyma of a leaf, located within the epidermis.

messenger RNA (mRNA) The RNA transcribed from structural genes; complementary to a portion of one strand of DNA.

metabolism The sum of all chemical processes occurring within a living cell or organism.

metacercaria An encysted form of a larval liver fluke, found in muscle tissue of an infected animal.

metamorphosis Process in which a marked change in form takes place during postembryonic development.

metaphase The stage of mitosis or meiosis during which chromosomes align in the spindle’s equatorial plane.

metastasis The process by which cancer cells move from their point of origin to other locations in the body.

methanogens Obligate, anaerobic archaebacteria that produce methane.

methylation The addition of a methyl group to bases in DNA; often correlated with DNA that is not expressed.

microbody A cellular organelle bounded by a single membrane, including peroxisomes and glyoxysomes.

microevolution Refers to the evolutionary process itself; evolution within a species.

microglia Neuroglial cells that serve as macrophages in the central nervous system.

micronucleus A diploid nucleus found in all ciliates used only in sexual reproduction.

micronutrient A mineral required in only minute amounts for plant growth, such as iron, chlorine, or copper.

microphyll In plants, a leaf that has only one vein connecting it to the vascular cylinder of the stem.

micropyle In the ovules of seed plants, an opening in the integuments through which the pollen tube enters.

micro-RNA (miRNA) A class of very short RNAs involved in controlling gene expression.

microtubule In eukaryotic cells, a long, hollow protein cylinder composed of tubulin.

microvillus Cytoplasmic projection from epithelial cells that increases surface area for absorption.

middle lamella The layer of intercellular material that cements together the primary walls of adjacent plant cells.

mimicry The resemblance in form, color, or behavior of certain organisms (mimics) to others (models).

miracidium The ciliated first-stage larva inside the egg of the liver fluke.

mismatch repair A form of DNA repair that restores replication errors.

missense mutation A base substitution mutation that results in the alteration of a single amino acid.

mitochondrion The organelle called the powerhouse of the cell; supports electron transport and ATP synthesis.

mitogen-activated protein (MAP) kinase Any of a class of protein kinases that activate transcription factors to alter gene expression.

mitosis Somatic cell division; nuclear division in which duplicated chromosomes separate into two identical nuclei.

molar concentration Concentration expressed as moles of a substance in 1 L of pure water.

mole The weight of a substance in grams that corresponds to the atomic masses of all component atoms (6.023 x 10^23 molecules).

molecular clock method In evolutionary theory, the method in which the rate of evolution of a molecule is constant through time.

molecular cloning The isolation and amplification of a specific sequence of DNA.

monocot Short for monocotyledon; flowering plant in which the embryos have only one cotyledon.

monocyte A type of leukocyte that becomes a phagocytic cell (macrophage) after moving into tissues.

monoecious A plant in which the staminate and pistillate flowers are separate, but borne on the same individual.

monokaryotic In fungus, describing a compartment containing only a single nucleus.

monomer The smallest chemical subunit of a polymer.

monophyletic In phylogenetic classification, a group that includes the most recent common ancestor and all its descendants.

monosaccharide A simple sugar that cannot be decomposed into smaller sugar molecules.

monosomic Describes the condition in which a chromosome has been lost due to nondisjunction during meiosis.

monotreme An egg-laying mammal.

morphogen A signal molecule produced by an embryonic organizer region that determines relative positions of cells.

morphogenesis The development of an organism’s body form, including its organs and anatomical features.

morphology The form and structure of an organism.

morula Solid ball of cells in the early stage of embryonic development.

mosaic development A pattern of embryonic development in which initial cells contain different developmental signals.

motif A substructure in proteins that confers function, such as the helix-turn-helix motif.

motor (efferent) neuron Neuron that transmits nerve impulses from the central nervous system to an effector.

M phase The phase of cell division during which chromosomes are separated.

M phase-promoting factor (MPF) A Cdk enzyme active at the G2/M checkpoint.

Müllerian mimicry A phenomenon in which two or more unrelated but protected species resemble one another.

multidrug-resistant (MDR) strain Any bacterial strain that has become resistant to more than one antibiotic drug.

multienzyme complex An assembly consisting of several enzymes catalyzing different steps in a sequence of reactions.

multigene family A collection of related genes on a single chromosome or on different chromosomes.

multipotent Describing a stem cell that can give rise to multiple different cells.

muscle fiber A long, cylindrical, multinucleated cell containing numerous myofibrils.

muscle spindle fiber Sensory organ that monitors the state of contraction within a skeletal muscle.

mutagen An agent that induces changes in DNA (mutations).

mutation A permanent change in a cell’s DNA, including nucleotide sequence changes or gene duplications.

mutualism A symbiotic association in which two (or more) organisms live together, and both members benefit.

mycelium In fungi, a mass of hyphae.

mycorrhiza A symbiotic association between fungi and the roots of a plant.

mycotoxins Chemicals produced by fungi that cause disease.

myelin sheath A fatty layer surrounding the long axons of motor neurons in vertebrates.

myofilament A contractile microfilament, composed largely of actin and myosin, within muscle.

myosin One of the two protein components of myofilaments; functions as a motor protein using ATP.

NAD+/NADH See nicotinamide adenine dinucleotide.

NADH dehydrogenase An enzyme that catalyzes the oxidation by NAD+ of pyruvate to acetyl-CoA.

nanos A gene in Drosophila involved in anterior–posterior specification.

natural killer cell A cell that does not kill invading microbes, but rather, kills the cells infected by them.

natural selection The differential reproduction of genotypes caused by factors in the environment.

nauplius A larval form characteristic of crustaceans.

necrosis Cell death caused by factors external to the cell, such as injury or toxins.

negative control A type of control at the level of DNA transcription initiation in which the frequency of initiation is decreased.

negative feedback A homeostatic control mechanism whereby an increase in some substance inhibits the process leading to the increase.

nematocyst A harpoonlike structure found in the cnidocytes of animals in the phylum Cnidaria.

nephridium In invertebrates, a tubular excretory structure.

nephrid organ A filtration system of many freshwater invertebrates.

nephron Functional unit of the vertebrate kidney.

nephrostome The funnel-shaped opening that leads to the nephridium in mollusks.

nerve A group or bundle of nerve fibers (axons) with accompanying neurological cells.

nerve cord One of the distinguishing features of chordates, running lengthwise beneath the embryo’s dorsal surface.

neural crest A migratory cell population that arises from the neural tube.

neural groove The long groove formed along the long axis of the embryo by a layer of ectodermal cells.

neural tube The dorsal tube, formed from the neural plate, that differentiates into the brain and spinal cord.

neuroglia Nonconducting nerve cells that are intimately associated with neurons and provide support.

neuromuscular junction The structure formed when the tips of axons contact (innervate) a muscle fiber.

neuron A nerve cell specialized for signal transmission; includes cell body, dendrites, and axon.

neurotransmitter A chemical released at the axon terminal of a neuron that travels across the synaptic cleft.

neurotropin Growth factors that are important regulators of the nervous system.

neurulation A process in early embryonic development by which a dorsal band of ectoderm thickens and rolls into the neural tube.

neutrophil An abundant type of granulocyte capable of engulfing microorganisms and other foreign particles.

niche The role played by a particular species in its environment.

nicotinamide adenine dinucleotide (NAD) A molecule that becomes reduced (to NADH) as it carries high-energy electrons from oxidized molecules.

nitrification The oxidization of ammonia or nitrite to produce nitrate.

nociceptor A naked dendrite that acts as a receptor in response to a pain stimulus.

nocturnal Active primarily at night.

node The part of a plant stem where one or more leaves are attached.

node of Ranvier A gap formed at the point where two Schwann cells meet on an axon.

nodule In plants, a specialized tissue that houses beneficial bacteria, such as nitrogen-fixing bacteria.

nonassociative learning A learned behavior that does not require an animal to form an association between two stimuli.

noncompetitive inhibitor An inhibitor that binds to a location other than the active site of an enzyme.

noncyclic photophosphorylation The set of light-dependent reactions of the two plant photosystems.

nondisjunction The failure of homologues or sister chromatids to separate during mitosis or meiosis.

nonextreme archaea Archaeal groups that are not extremophiles, living in more moderate environments.

nonpolar Said of a covalent bond that involves equal sharing of electrons.

nonsense codon One of three codons (UAA, UAG, and UGA) that serve as “stop” signals in the mRNA message.

nonsense mutation A base substitution in which a codon is changed into a stop codon.

Northern blot A blotting technique used to identify a specific mRNA sequence in a complex mixture.

notochord In chordates, a dorsal rod of cartilage that runs the length of the body.

nucellus Tissue composing the chief pair of young ovules; equivalent to a megasporangium.

nuclear envelope The bounding structure of the eukaryotic nucleus.

nuclear pore Tiny but complex openings in the nuclear envelope that allow selective passage of proteins and nucleic acids.

nuclear receptor Intracellular receptors found in both the cytoplasm and the nucleus.

nucleic acid A nucleotide polymer; chief types are DNA and RNA.

nucleoid The area of a prokaryotic cell that contains the genome.

nucleolus In eukaryotes, the site of rRNA synthesis.

nucleosome A complex consisting of a DNA duplex wound around a core of eight histone proteins.

nucleotide A single unit of nucleic acid, composed of a phosphate, a 5-carbon sugar, and a nitrogenous base.

nucleus The central core of an atom; in eukaryotic cells, the membranous organelle that houses DNA.

nutritional mutation A mutation affecting a synthetic pathway for a vital compound, such as an amino acid.

ocellus A simple light receptor common among invertebrates.

octet rule Rule to describe patterns of chemical bonding in main group elements that require a total of eight electrons to complete their outer electron shell.

Okazaki fragment A short segment of DNA produced by discontinuous replication elongating in the 5′-to-3′ direction away from the replication.

olfaction The function of smelling.

oligodendrocytes Neuroglial cells that produce myelin in the central nervous system.

ommatidium The visual unit in the compound eye of arthropods; contains light-sensitive cells and a lens able to form an image.

oncogene A mutant form of a growth-regulating gene that is inappropriately “on,” causing unrestrained cell growth and division.

oocyst The zygote in a sporozoan life cycle. It is surrounded by a tough cyst to prevent dehydration or other damage.

open circulatory system A circulatory system in which the blood flows into sinuses in which it mixes with body fluid and then reenters the vessels in another location.

open reading frame (ORF) A region of DNA that encodes a sequence of amino acids with no stop codons in the reading frame.

operant conditioning A learning mechanism in which the reward follows only after the correct behavioral response.

operator A regulatory site on DNA to which a repressor can bind to prevent or decrease initiation of transcription.

operculum A flat, bony, external protective covering over the gill chamber in fish.

operon A cluster of adjacent structural genes transcribed as a unit into a single mRNA molecule.

opisthosoma The posterior portion of the body of an arachnid.

oral surface The surface on which the mouth is found; used as a reference when describing the body structure of echinoderms.

orbital A region around the nucleus of an atom with a high probability of containing an electron.

order A category of classification above the level of family and below that of class.

organ A body structure composed of several different tissues grouped in a structural and functional unit.

organelle Specialized part of a cell; literally, a small cytoplasmic organ.

orthologues Genes that reflect the conservation of a single gene found in an ancestor.

oscillating selection The situation in which selection alternately favors one phenotype at one time, and a different phenotype at a another time.

osculum A specialized, larger pore in sponges through which filtered water is forced to the outside of the body.

osmoconformer An animal that maintains the osmotic concentration of its body fluids at about the same level as that of the medium in which it is living.

osmolarity A measure of the number of osmotically active moles of a solute per liter of solution.

osmosis The diffusion of water across a selectively permeable membrane.

osmotic pressure The potential pressure developed by a solution separated from pure water by a differentially permeable membrane.

ossicle Any of a number of movable or fixed calcium-rich plates that collectively make up the endoskeleton of echinoderms.

osteoblast A bone-forming cell.

osteocyte A mature osteoblast.

outcrossing Breeding with individuals other than oneself or one’s close relatives.

outgroup A species or group of species that is closely related to, but not a member of, the group under study.

ovary (1) In animals, the organ in which eggs are produced. (2) In flowering plants, the enlarged basal portion of a carpel that contains the ovule(s).

oviduct In vertebrates, the passageway through which ova (eggs) travel from the ovary to the uterus.

oviparity Refers to a type of reproduction in which the eggs are developed after leaving the body of the mother.

ovoviviparity Refers to a type of reproduction in which young hatch from eggs that are retained in the mother’s uterus.

ovulation In animals, the release of an egg or eggs from the ovary.

ovule The sporophyte structure that produces and contains female reproductive cells in angiosperms.

ovum The egg cell; female gamete.

oxidation Loss of an electron by an atom or molecule; in metabolism, often associated with a gain of oxygen or a loss of hydrogen.

oxidation–reduction reaction A type of paired reaction in living systems in which electrons lost from one atom are gained by another atom.

oxidative phosphorylation Synthesis of ATP by ATP synthase using energy from a proton gradient.

oxygen debt The amount of oxygen required to convert the lactic acid generated in the muscles during exercise back into glucose.

oxytocin A hormone of the posterior-pituitary gland that affects uterine contractions during childbirth and stimulates lactation.

ozone O3, a stratospheric layer of the Earth’s atmosphere responsible for filtering out ultraviolet radiation supplied by the Sun.

p53 gene The gene that produces the p53 protein that monitors DNA integrity and halts cell division if DNA damage is detected.

pacemaker A patch of excitatory tissue in the vertebrate heart that initiates the heartbeat.

paclitaxel (Taxol) A secondary metabolite produced in the Pacific yew tree that binds to and stabilizes microtubules.

pair-rule gene Any of certain genes in Drosophila development controlled by the gap genes that are expressed in stripes.

paleopolyploid An ancient polyploid organism used in analysis of polyploidy events in the study of a species’ genome evolution.

palisade parenchyma In plant leaves, the columnar, chloroplast-containing parenchyma cells of the mesophyll.

panspermia The hypothesis that meteors or cosmic dust may have brought significant amounts of complex organic molecules to Earth.

papilla A small projection of tissue.

paracrine A type of chemical signaling between cells in which the effects are local and short-lived.

paralogues Two genes within an organism that arose from the duplication of one gene in an ancestor.

paraphyletic In phylogenetic classification, a group that includes the most recent common ancestor of the group, but not all its descendants.

parapodia One of the paired lateral processes on each side of most segments in polychaete annelids.

parasexuality In certain fungi, the fusion and segregation of heterokaryotic haploid nuclei to produce recombinant nuclei.

parasitism A living arrangement in which an organism lives on or in an organism of a different species and derives nutrients from it.

parenchyma cell The most common type of plant cell; characterized by large vacuoles, thin walls, and functional nuclei.

parental investment The energy and time each sex invests in producing and rearing offspring.

parthenogenesis The development of an egg without fertilization, as in aphids, bees, ants, and some lizards.

partial diploid Describes an E. coli cell that carries an F′ plasmid with host genes.

partial pressure The components of each individual gas—such as nitrogen, oxygen, and carbon dioxide—that together constitute the total air pressure.

passive transport The movement of substances across a cell’s membrane without the expenditure of energy.

pathogen An infectious agent, usually a microorganism, capable of causing disease in an infected host.

pedigree A consistent graphic representation of matings and offspring over multiple generations for a particular genetic trait.

pedipalps A pair of specialized appendages found in arachnids.

pelagic Free-swimming, usually in open water.

pellicle A tough, flexible covering in ciliates and euglenoids.

pentaradial symmetry The five-part radial symmetry characteristic of adult echinoderms.

peptide bond The type of bond that links amino acids together in proteins through a dehydration reaction.

peptidoglycan A component of the cell wall of bacteria, consisting of carbohydrate polymers linked by protein cross-bridges.

peptidyl transferase In translation, the enzyme responsible for catalyzing the formation of a peptide bond between each new amino acid.

perennial Plants that grow year after year versus growing, reproducing, and dying in a single year.

perianth In flowering plants, the petals and sepals taken together.

pericycle In vascular plants, one or more cell layers surrounding the vascular tissues of the root.

periderm Outer protective tissue in vascular plants that is produced by the cork cambium.

peristalsis In animals, a series of alternating contracting and relaxing muscle movements along the length of a tube.

peroxisome A microbody that plays an important role in the breakdown of highly oxidative hydrogen peroxide by catalase.

petal A flower part, usually conspicuously colored; one of the units of the corolla.

petiole The stalk of a leaf.

phage conversion The phenomenon by which DNA from a virus, incorporated into a host cell’s genome, alters the host cell’s function.

phage lambda A well-known bacteriophage that has been widely used in genetic studies.

phagocyte Any cell that engulfs and devours microorganisms or other particles.

phagocytosis Endocytosis of a solid particle; the plasma membrane folds inward around the particle and engulfs it.

pharyngeal pouches In chordates, embryonic regions that become pharyngeal slits in aquatic chordates but do not develop openings in terrestrial vertebrates.

pharyngeal slits A group of openings on each side of the anterior region that form a passageway from the pharynx to the environment.

pharynx A muscular structure lying posterior to the mouth in many animals; aids in propelling food.

phase change The developmental transition seen in plants associated with an ability to flower.

phenotype The realized expression of the genotype; the physical appearance or functional expression of a trait.

pheromone Chemical substance released by one organism that influences the behavior or physiological processes of another of the same species.

phloem In vascular plants, a food-conducting tissue basically composed of sieve elements and various kinds of parenchyma cells.

phoronid Any of a group of lophophorate invertebrates that burrows into soft underwater substrates.

phosphatase Any of a number of enzymes that removes a phosphate group from a protein.

phosphodiester bond The linkage between two sugars in the backbone of a nucleic acid molecule.

phospholipid Similar in structure to a fat, but having only two fatty acids attached to the glycerol backbone.

phospholipid bilayer The main component of cell membranes; phospholipids naturally associate in a bilayer.

phosphorylation Chemical reaction resulting in the addition of a phosphate group to an organic molecule.

photic zone The area in an aquatic habitat that receives sufficient light for photosynthesis to occur.

photoelectric effect The ability of a beam of light to excite electrons, creating an electrical current.

photomorphogenesis Nondirectional, light-triggered development in plants.

photon A particle of light having a discrete amount of energy.

photoperiodism The tendency of biological reactions to respond to the duration and timing of day and night.

photoreceptor A light-sensitive sensory cell.

photorespiration Action of the enzyme rubisco, which catalyzes the oxidization of RuBP, releasing CO2.

photosystem An organized complex of chlorophyll, other pigments, and proteins that traps light energy as excited electrons.

phototropism In plants, a growth response to a light stimulus.

pH scale A scale used to measure acidity and basicity defined as the negative log of H+ concentration.

phycobiliprotein A type of accessory pigment found in cyanobacteria and some algae.

phycologist A scientist who studies algae.

phyllotaxy In plants, a spiral pattern of leaf arrangement on a stem.

phylogenetic species concept (PSC) The concept that defines species on the basis of their phylogenetic relationships.

phylogenetic tree A pattern of descent generated by analysis of similarities and differences among organisms.

phylogeny The evolutionary history of an organism, including which species are closely related and in what order.

phylum A major category, between kingdom and class, of taxonomic classifications.

physical map A map of the DNA sequence of a chromosome or genome based on actual landmarks.

phytoalexin An antimicrobial compound produced at the site of infection in plants.

phytochrome A plant pigment that is associated with the absorption of light; photoreceptor for red to far-red light.

phytoestrogen One of a number of secondary metabolites in some plants that are structurally similar to estrogen.

phytoremediation The process that uses plants to remove contamination from soil or water.

pigment A molecule that absorbs light.

pilus Extensions of a bacterial cell enabling it to transfer genetic materials.

pinocytosis The process of fluid uptake by endocytosis in a cell.

pistil Central organ of flowers, typically consisting of ovary, style, and stigma.

pith The ground tissue occupying the center of the stem or root within the vascular cylinder.

pituitary gland Endocrine gland at the base of the hypothalamus composed of anterior and posterior lobes.

placenta In mammals, a tissue through which the embryo is nourished while in the uterus.

plankton Free-floating, mostly microscopic, aquatic organisms.

plant receptor kinase Any of a group of plant membrane receptors that phosphorylate serine or threonine.

planula A ciliated, free-swimming larva produced by the medusae of cnidarian animals.

plasma The fluid of vertebrate blood; contains dissolved salts, metabolic wastes, hormones, and proteins.

plasma cell An antibody-producing cell resulting from the differentiation of a B lymphocyte.

plasma membrane The membrane surrounding the cytoplasm of a cell; consists of a single phospholipid bilayer.

plasmid A small fragment of extrachromosomal DNA, usually circular.

plasmodesmata In plants, cytoplasmic connections between adjacent cells.

plasmodium Stage in the life cycle of myxomycetes; a multinucleate mass of protoplasm.

plasmolysis The shrinking of a plant cell in a hypertonic solution.

plastid An organelle in the cells of photosynthetic eukaryotes that is the site of photosynthesis.

platelet-derived growth factor (PDGF) A signaling molecule (growth factor) that can stimulate cell division in fibroblasts.

platelet In mammals, a fragment of a white blood cell that functions in the formation of blood clots.

pleiotropy Condition in which an individual allele has more than one effect on production of the phenotype.

plesiomorphy In cladistics, another term for an ancestral character state.

plumule The epicotyl of a plant with its two young leaves.

pluripotent Describing a cell that can give rise to any cell in the body of an organism.

point mutation An alteration of one nucleotide in a chromosomal DNA molecule.

polar body Minute, nonfunctioning cell produced during the meiotic divisions leading to gamete formation.

polar covalent bond A covalent bond in which electrons are shared unequally.

polarity (1) Refers to unequal charge distribution in a molecule. (2) Refers to axial differences in a developing embryo.

polarize In cladistics, to determine whether character states are ancestral or derived.

pollen tube A tube formed after germination of the pollen grain; carries the male gametes into the ovule.

pollination The transfer of pollen from an anther to a stigma.

polyandry The condition in which a female mates with more than one male.

polyclonal antibody An antibody response in which an antigen elicits many different antibodies.

polygenic inheritance Describes a mode of inheritance in which more than one gene affects a trait.

polygyny A mating choice in which a male mates with more than one female.

polymer A molecule composed of many similar or identical molecular subunits.

polymerase chain reaction (PCR) A process by which DNA polymerase is used to copy a sequence of interest repeatedly.

polymorphism The presence in a population of more than one allele of a gene.

polyp A typically sessile, cylindrical body form found in cnidarian animals.

polypeptide A molecule consisting of many joined amino acids.

polyphyletic In phylogenetic classification, a group that does not include the most recent common ancestor.

polyploidy Condition in which one or more entire sets of chromosomes is added to the diploid genome.

polysaccharide A carbohydrate composed of many monosaccharide sugar subunits linked together.

polyunsaturated fat A fat molecule having at least two double bonds between adjacent carbons.

population Any group of individuals, usually of a single species, occupying a given area.

population genetics The study of the properties of genes in populations.

positive control A type of control at the level of DNA transcription initiation in which the frequency of initiation is increased.

posttranscriptional control A mechanism of control over gene expression that operates after the transcription of mRNA.

postzygotic isolating mechanism A type of reproductive isolation in which zygotes are produced but are unable to develop.

potential energy Energy that is not being used, but could be; energy in a potentially usable form.

precapillary sphincter A ring of muscle that guards each capillary loop.

pre-mRNA splicing In eukaryotes, the process by which introns are removed from the primary transcript.

pressure potential In plants, the turgor pressure resulting from pressure against the cell wall.

pressure–flow hypothesis A hypothesis explaining how phloem moves from a source to a sink in plants.

prezygotic isolating mechanism A type of reproductive isolation in which the formation of a zygote is prevented.

primary endosperm nucleus In flowering plants, the result of the fusion of a sperm nucleus and the (usually) two polar nuclei.

primary growth In vascular plants, growth originating in the apical meristems of shoots and roots.

primary immune response The first response of an immune system to a foreign antigen.

primary induction Inductions between the three primary tissue types: mesoderm and endoderm.

primary meristem Any of the three meristems produced by the apical meristem.

primary nondisjunction Failure of chromosomes to separate properly at meiosis I.

primary phloem The cells involved in food conduction in plants.

primary plant body The part of a plant consisting of young, soft shoots and roots derived from apical meristem.

primary productivity The amount of energy produced by photosynthetic organisms in a community.

primary structure The specific amino acid sequence of a protein.

primary tissues Tissues that make up the primary plant body.

primary transcript The initial mRNA molecule copied from a gene by RNA polymerase.

primary wall In plants, the wall layer deposited during the period of cell expansion.

primase The enzyme that synthesizes the RNA primers required by DNA polymerases.

primate Monkeys and apes (including humans).

primitive streak In the early embryos of birds, reptiles, and mammals, a dorsal, longitudinal strip of ectoderm and mesoderm.

primordium In plants, a bulge on the young shoot produced by the apical meristem.

principle of parsimony Principle stating that scientists should favor the hypothesis that requires the fewest assumptions.

prions Infectious proteinaceous particles.

procambium In vascular plants, a primary meristematic tissue that gives rise to primary vascular tissues.

product rule See rule of multiplication.

proglottid A repeated body segment in tapeworms.

prokaryote A cell lacking a membrane-bounded nucleus or membrane-bounded organelles.

prometaphase The transitional phase between prophase and metaphase.

promoter A DNA sequence that provides a recognition and attachment site for RNA polymerase.

prophage A viral genome stably integrated into a host cell’s genome.

prophase The phase of cell division that begins when the condensed chromosomes become visible.

proprioceptor In vertebrates, a sensory receptor that senses the body’s position and movements.

prosimian Any member of the mammalian group that is a sister group to the anthropoids.

prosoma The anterior portion of the body of an arachnid.

prostaglandins A group of modified fatty acids that function as chemical messengers.

prostate gland In male mammals, a mass of glandular tissue at the base of the urethra.

protease An enzyme that degrades proteins by breaking peptide bonds.

proteasome A large, cylindrical cellular organelle that degrades proteins marked with ubiquitin.

protein A chain of amino acids joined by peptide bonds.

protein kinase An enzyme that adds phosphate groups to proteins.

protein microarray An array of proteins on a microscope slide or silicon chip.

proteome All the proteins coded for by a particular genome.

proteomics The study of the proteomes of organisms.

protoderm The primary meristem that gives rise to the dermal tissue.

proton pump A protein channel in a membrane of the cell that expends energy to transport protons.

proto-oncogene A normal cellular gene that can act as an oncogene when mutated.

protostome Any member of a grouping of bilaterally symmetrical animals in which the mouth develops first.

proximal convoluted tubule The initial portion of the renal tubule system of the kidney.

pseudocoel A body cavity located between the endoderm and mesoderm.

pseudocoelom A body cavity that develops embryologically between the mesoderm and endoderm.

pseudogene A copy of a gene that is not transcribed.

pseudomurien A component of the cell wall of archaea.

pseudopod A nonpermanent cytoplasmic extension of the cell body.

P site In a ribosome, the peptidyl site that binds to the tRNA attached to the growing polypeptide chain.

punctuated equilibrium A hypothesis about the mechanism of evolutionary change proposing periods of rapid evolution.

Punnett square A diagrammatic way of showing the possible genotypes and phenotypes of genetic crosses.

pupa A developmental stage of some insects in which the organism is nonfeeding and immotile.

purine The larger of the two general kinds of nucleotide base found in DNA and RNA.

pyrimidine The smaller of the two general kinds of nucleotide base found in DNA and RNA.

pyruvate A 3-carbon molecule that is the end product of glycolysis.

quantitative trait A trait that is determined by the effects of more than one gene.

quaternary structure The structural level of a protein composed of more than one polypeptide chain.

quinine A compound found in the bark of Cinchona trees used to treat malaria.

radial canal Any of five canals that connect to the ring canal of an echinoderm’s water–vascular system.

radial cleavage The embryonic cleavage pattern of deuterostome animals in which cells divide parallel to and at right angles to the polar axis.

radial symmetry A type of structural symmetry with a circular plan, such that dividing the body through the midpoint in any direction yields two identical sections.

radicle The part of the plant embryo that develops into the root.

radioactive isotope An isotope that is unstable and undergoes radioactive decay, releasing energy.

radioactivity The emission of nuclear particles and rays by unstable atoms as they decay into more stable forms.

radula Rasping tongue found in most mollusks.

Ras A member of an extensive family of small G proteins involved in cell signaling; links a membrane RTK with a kinase cascade.

reaction center A transmembrane protein complex in a photosystem that receives energy from the antenna complex, exciting an electron.

reading frame The correct succession of nucleotides in triplet codons that specify amino acids on translation.

realized niche The actual niche occupied by an organism when all biotic and abiotic interactions are taken into account.

receptor-mediated endocytosis Process by which specific macromolecules are transported into eukaryotic cells at clathrin-coated pits.

receptor protein A cellular protein that can bind to a ligand and initiate a signal transduction pathway.

receptor tyrosine kinase (RTK) A diverse group of membrane receptors that when activated have kinase enzymatic activity, phosphorylating proteins on tyrosine.

recessive An allele that is expressed only when present in the homozygous condition.

reciprocal altruism Performance of an altruistic act with the expectation that the favor will be returned.

reciprocal cross A genetic cross involving a single trait in which the sex of the parents is reversed.

reciprocal recombination A mechanism of genetic recombination in which two chromosomes trade segments.

recombinant DNA Fragments of DNA from two different species joined together in the laboratory into a single molecule.

recombination frequency The value obtained by dividing the number of recombinant progeny by the total progeny in a test cross.

redia A secondary, nonciliated larva produced in the sporocysts of liver flukes.

reduction The gain of an electron by an atom, often with an associated proton.

reflex In the nervous system, a motor response subject to little associative modification.

reflex arc The nerve path in the body that leads from stimulus to reflex action.

refractory period The recovery period after membrane depolarization during which the membrane is unable to respond to stimulation.

regulatory protein Any of a group of proteins that modulates the ability of RNA polymerase to bind to a promoter.

reinforcement In speciation, the process by which partial reproductive isolation between populations is increased by selection against mating.

replica plating A method of transferring bacterial colonies from one plate to another to make a copy of the original plate.

replication fork The Y-shaped end of a growing replication bubble in a DNA molecule.

replicon An origin of DNA replication and the DNA whose replication is controlled by this origin.

replisome The macromolecular assembly of enzymes involved in DNA replication.

repolarization Return of the ions in a nerve to their resting potential distribution following depolarization.

repression Control of gene expression by preventing transcription, mediated by repressor proteins.

repressor A protein that regulates DNA transcription by preventing RNA polymerase from attaching to the promoter.

reproductive isolating mechanism Any barrier that prevents genetic exchange between species.

reproductively isolated Describing populations whose members do not mate with each other or who cannot produce fertile offspring.

residual volume The amount of air remaining in the lungs after the maximum amount of air has been exhaled.

resting membrane potential The charge difference (electric potential) that exists across a neuron at rest (about 70 mV).

restriction endonuclease An enzyme that cleaves a DNA duplex molecule at a particular base sequence; also called a restriction enzyme.

restriction fragment length polymorphism (RFLP) A technique where restriction enzymes recognize specific DNA sequences to create different-length fragments.

retina The photosensitive layer of the vertebrate eye containing neurons and light receptors (rods and cones).

retinoblastoma susceptibility gene (Rb) A gene that, when mutated, predisposes individuals to a rare form of cancer of the retina.

retrovirus An RNA virus that transcribes its RNA into duplex DNA using reverse transcriptase.

reverse genetics An approach where a researcher uses a cloned gene of unknown function and creates a mutation to assess its effects.

reverse transcriptase A viral enzyme found in retroviruses capable of converting their RNA genome into a DNA copy.

Rh blood group A set of cell-surface markers (antigens) on red blood cells; grouped into Rh-positive and Rh-negative.

rhizome In vascular plants, a more or less horizontal underground stem used for storage or vegetative reproduction.

rhynchocoel A true coelomic cavity in ribbonworms that serves as a hydraulic power source for the proboscis.

ribonucleic acid (RNA) A class of nucleic acids characterized by the presence of ribose and the pyrimidine uracil.

ribosomal RNA (rRNA) A class of RNA molecules found in ribosomes; transcribed from the DNA of the nucleolus.

ribosome The molecular machine that carries out protein synthesis.

ribosome-binding sequence (RBS) In prokaryotes, a conserved sequence at the 5' end of mRNA that helps position the ribosome.

ribozyme An RNA molecule that can behave as an enzyme.

ribulose 1,5-bisphosphate (RuBP) In the Calvin cycle, the 5-carbon sugar to which CO2 is attached.

ribulose bisphosphate carboxylase/oxygenase (rubisco) The four-subunit enzyme in the chloroplast that catalyzes carbon fixation.

RISC (RNA-induced silencing complex) A complex of protein and RNA that can affect gene expression by inhibiting translation.

RNA interference A type of gene silencing where mRNA transcripts are prevented from being translated.

RNA polymerase An enzyme that catalyzes the assembly of an mRNA molecule from a DNA template.

RNA primer A sequence of about 10 RNA nucleotides that attaches at a replication fork to start DNA synthesis.

RNA splicing A nuclear process where intron sequences are cut out and exon sequences are spliced together.

rod Light-sensitive nerve cell in the vertebrate retina responsible for "night vision."

root The usually descending axis of a plant that anchors it and absorbs water and minerals.

root cap A tissue structure at the growing tips of roots that protects the root apical meristem.

root hair A tubular extension from an epidermal cell that increases surface area for absorption.

root pressure Pressure exerted by water in the roots in response to a solute potential.

root system The portion of the plant body that anchors the plant and absorbs water.

R plasmid A resistance plasmid that picks up antibiotic resistance genes.

rule of addition The rule stating that the probability of either of two independent events occurring is the sum of their individual probabilities.

rule of multiplication The rule stating that the probability of two independent events occurring is the product of their individual probabilities.

rumen An "extra stomach" in cows where cellulose digestion occurs.

salicylic acid In plants, an organic molecule that is a long-distance signal in systemic acquired resistance.

saltatory conduction A very fast form of nerve impulse conduction where impulses leap from node to node.

sap The fluid contents of the xylem or phloem vessels.

saprobes Heterotrophic organisms that digest their food externally (e.g., most fungi).

sarcolemma The specialized cell membrane in a muscle cell.

sarcomere The fundamental unit of contraction in skeletal muscle; repeating bands of actin and myosin.

sarcoplasmic reticulum The endoplasmic reticulum of a muscle cell.

satellite DNA A nontranscribed region of the chromosome with a short nucleotide sequence repeated tandemly.

saturated fat A fat composed of fatty acids in which all internal carbon atoms contain the maximum possible number of hydrogen atoms.

Schwann cells Supporting cells that make up the peripheral nervous system.

sclereid In vascular plants, a sclerenchyma cell with a thick, lignified secondary wall.

sclerenchyma cell Tough, thick-walled cells that strengthen plant tissues.

scolex The attachment organ at the anterior end of a tapeworm.

scrotum The pouch that contains the testes in most mammals.

scuttellum The modified cotyledon in cereal grains.

second filial (F2) generation The offspring resulting from a cross between members of the F1 generation.

secondary cell wall The innermost layer of the cell wall in plants, often impregnated with lignin.

secondary growth In vascular plants, an increase in stem and root diameter caused by lateral meristems.

secondary immune response The swifter response of the body the second time it is invaded by the same pathogen.

secondary induction An induction between tissues that have already differentiated.

secondary metabolite A molecule not directly involved in growth or reproduction (e.g., nicotine, caffeine).

secondary plant body The part of a plant consisting of secondary tissues from lateral meristems.

secondary structure In a protein, hydrogen-bonding interactions such as the alpha helix or beta sheet.

secondary tissue Any tissue formed from lateral meristems in trees and shrubs.

Second Law of Thermodynamics Statement that disorder (entropy) is continually increasing in the universe.

second messenger A small molecule or ion that carries the message from a receptor to the cytoplasm.

seed bank Ungerminated seeds in the soil of an area.

seed coat In plants, the outer layers of the ovule that protect the dormant embryo.

segmentation The division of the developing animal body into repeated units.

segmentation gene Any of the three classes of genes that control development of the segmented body plan of insects.

segment polarity gene Any of certain genes in Drosophila development that subdivide the stripes created by pair-rule genes.

segregation The process by which alternative forms of traits are expressed in offspring rather than blending.

selection The process by which some organisms leave more offspring than others based on genetic traits.

selectively permeable Condition in which a membrane is permeable to some substances but not others.

self-fertilization The union of egg and sperm produced by a single hermaphroditic organism.

self-incompatibility A genetic system in plants that prevents self-fertilization.

semen In reptiles and mammals, sperm-bearing fluid expelled during orgasm.

semicircular canal Any of three fluid-filled canals in the inner ear that help maintain balance.

semiconservative replication DNA replication where each strand of the original duplex serves as a template for a new strand.

senescent Aged, or in the process of aging.

sensory (afferent) neuron A neuron that transmits nerve impulses from a sensory receptor to the central nervous system.

sensory exploitation Evolution in males of a signal that exploits preexisting sensory biases in females.

sensory setae In insects, bristles attached to the nervous system that are sensitive to mechanical stimulation.

sepal A member of the outermost floral whorl of a flowering plant.

septate junction A type of tight junction between animal cells that forms a waterproof barrier.

septation In prokaryotic cell division, the formation of a septum where new cell wall is formed.

septum A wall between two cavities.

sequence-tagged site (STS) A small stretch of DNA that is unique in a genome used as a physical marker.

seta In an annelid, bristles of chitin that help anchor the worm during locomotion.

severe acute respiratory syndrome (SARS) A respiratory infection with an 8% mortality rate caused by a coronavirus.

sex chromosome A chromosome that is related to sex (X and Y chromosomes).

sex-linked A trait determined by a gene carried on the X chromosome and absent on the Y chromosome.

sexual dimorphism Morphological differences between the sexes of a species.

sexual reproduction The process of producing offspring through alternation of fertilization and meiotic reduction.

sexual selection A type of differential reproduction that results from variable success in obtaining mates.

shared derived character In cladistics, character states that are shared by species and different from the ancestral state.

shoot In vascular plants, the aboveground portions (stems and leaves).

short interspersed element (SINE) Any of a type of retrotransposon found in humans and other primates.

shotgun sequencing Method of DNA sequencing where DNA is randomly cut, cloned, and sequenced.

sieve cell In the phloem of vascular plants, a long, slender element with relatively unspecialized sieve areas.

signal recognition particle (SRP) A cytoplasmic complex of proteins that recognizes and binds to the signal sequence of a polypeptide.

signal sequence A sequence in the 5' end of an mRNA that targets the mRNA to the RER for translation.

signal transduction The events that occur within a cell on receipt of a signal.

single-nucleotide polymorphism (SNP) A site present in at least 1% of the population at which individuals differ by a single nucleotide.

single nucleotide variation (SNV) DNA variation in a population that affects a single site in DNA.

sinus A cavity or space in tissues or in bone.

sister chromatid One of two identical copies of each chromosome produced for mitotic division.

small interfering RNAs (siRNAs) A class of micro-RNAs involved in control of gene transcription.

small nuclear ribonucleoprotein particles (snRNP) In eukaryotes, a complex that clusters to form the spliceosome.

small nuclear RNA (snRNA) In eukaryotes, a small RNA sequence that facilitates recognition and excision of introns.

sodium–potassium pump Transmembrane channels engaged in the active transport of Na+ and K+.

solute A molecule dissolved in some solution.

solute potential The amount of osmotic pressure arising from the presence of a solute in water.

solvent The medium in which one or more solutes is dissolved.

somatic cell Any of the cells of a multicellular organism except those destined to form gametes.

somatic cell nuclear transfer (SCNT) The transfer of the nucleus of a somatic cell into an enucleated egg cell.

somatic mutation A change in genetic information occurring in somatic cells, not passed to the next generation.

somatic nervous system In vertebrates, the neurons of the peripheral nervous system that control skeletal muscle.

somite One of the blocks of tissue into which the mesoderm is divided during embryonic differentiation.

Southern blot A technique in which DNA fragments are separated by gel electrophoresis and blotted.

S phase The phase of the cell cycle during which DNA replication occurs.

specialized transduction The transfer of only a few specific genes into a bacterium using a lysogenic bacteriophage.

speciation The process by which new species arise.

species A kind of organism; designated by binomial names written in italics.

specific heat The amount of heat that must be absorbed or lost by 1 g of a substance to raise its temperature 1°C.

specific transcription factor Any of a great number of transcription factors that act in a time- or tissue-dependent manner.

spectrin A scaffold of proteins that links plasma membrane proteins to actin filaments.

sperm competition Competition among sperm to successfully fertilize a female’s egg.

spermatid In animals, each of four haploid cells that result from the meiotic divisions of a spermatocyte.

spermatozoa The male gamete, usually smaller than the female gamete and motile.

sphincter In vertebrate animals, a ring-shaped muscle capable of closing a tubular opening.

sphingolipid A phospholipid built on the carbon skeleton of the hydrocarbon ceramide.

spicule Any of a number of minute needles of silica or calcium carbonate found in sponges.

spindle apparatus The assembly of microtubules that separation chromosomes during cell division.

spindle checkpoint The third cell-division checkpoint that ensures all chromosomes are attached to the spindle.

spinnerets Organs at the posterior end of a spider’s abdomen that secrete silk.

spiracle External opening of a trachea in arthropods.

spiral cleavage The embryonic cleavage pattern where cells divide at an angle oblique to the polar axis.

spiralian A member of a group of invertebrate animals, many of which exhibit spiral cleavage.

spliceosome In eukaryotes, a complex responsible for excision of introns and joining of exons.

spongin A tough protein made by many kinds of sponges.

spongy parenchyma A leaf tissue composed of loosely arranged, chloroplast-bearing cells.

sporangium A structure in which spores are produced.

spore A haploid reproductive cell capable of developing into an adult without fusion.

sporophyte The spore-producing, diploid (2n) phase in the life cycle of a plant.

stabilizing selection A form of selection in which selection acts to eliminate both extremes from a range of phenotypes.

stamen The organ of a flower that produces pollen; consists of anther and filament.

starch An insoluble polymer of glucose; the chief food storage substance of plants.

start codon The AUG triplet which indicates the site of the beginning of mRNA translation.

stasis A period of time during which little evolutionary change occurs.

statocyst Sensory receptor sensitive to gravity and motion.

stele The central vascular cylinder of stems and roots.

stem cell A relatively undifferentiated cell in animal tissue that can divide to produce more differentiated cells.

stereoscopic vision Ability to perceive a single, three-dimensional image from slightly divergent two-dimensional images.

stigma In angiosperm flowers, the region of a carpel that serves as a receptive surface for pollen grains.

stipules Leaflike appendages that occur at the base of some flowering plant leaves.

stolon A stem that grows horizontally along the ground surface and may form adventitious roots.

stoma In plants, a minute opening bordered by guard cells in the epidermis.

stop codon Any of the three codons (UAA, UAG, and UGA) that indicate where mRNA translation is to be terminated.

STR (short tandem repeat) A form of genetic variation involving differences in repeated sequences of 2 to 4 bases.

stratify To hold plant seeds at a cold temperature for a certain period to trigger germination.

stratum corneum The outer layer of the epidermis of the skin of the vertebrate body.

striated muscle Skeletal voluntary muscle and cardiac muscle.

stroma In chloroplasts, the semiliquid substance that surrounds the thylakoid system.

stromatolite A fossilized mat of ancient bacteria.

style In flowers, the slender column of tissue through which the pollen tube grows.

stylet A piercing organ, usually a mouthpart, in some invertebrates.

suberin In plants, a fatty acid chain that forms the impermeable barrier in the Casparian strip.

subspecies A geographically defined population within a single species that has distinctive characteristics.

substrate The foundation to which an organism is attached, or a molecule on which an enzyme acts.

succession In ecology, the slow, orderly progression of changes in community composition.

summation Repetitive activation of the motor neuron resulting in maximum sustained contraction.

supercoiling The coiling in space of double-stranded DNA molecules due to torsional strain.

surface area-to-volume ratio Relationship of the surface area of a structure to the volume it contains.

surface tension A tautness of the surface of a liquid caused by cohesion of molecules.

suspensor In plants, a nutrient conduit from maternal tissue to the embryo.

swim bladder An organ in bony fish that helps regulate buoyancy.

swimmerets In lobsters and crayfish, appendages used in swimming and reproduction.

symbiosis The condition in which two or more dissimilar organisms live together in close association.

sympatric speciation The differentiation of populations within a common geographic area into species.

symplast route In plant roots, the pathway for movement of water and minerals through plasmodesmata.

symplesiomorphy In cladistics, another term for a shared ancestral character state.

symporter A carrier protein that transports two molecules in the same direction across the membrane.

synapomorphy In systematics, a derived character that is shared by clade members.

synapse A junction between a neuron and another cell where signals are bridged by neurotransmitters.

synapsid Any of an early group of reptiles that had a pair of temporal openings in the skull behind the eye sockets.

synapsis The point-by-point alignment of homologous chromosomes that occurs before the first meiotic division.

synaptic cleft The space between two adjacent neurons.

synaptic vesicle A vesicle of a neurotransmitter produced by the axon terminal of a nerve.

synaptonemal complex A protein lattice that forms between two homologous chromosomes in prophase I of meiosis.

syncytial blastoderm A structure composed of a single large cytoplasm containing about 4000 nuclei.

syngamy The process by which two haploid cells fuse to form a diploid zygote; fertilization.

synthetic polyploid A polyploid organism created by crossing organisms closely related to an ancestral species.

systematics The reconstruction and study of evolutionary relationships.

systemic acquired resistance (SAR) In plants, a longer-term response to a pathogen or pest attack.

systemin In plants, an 18-amino-acid peptide produced by damaged leaves that leads to the wound response.

systolic pressure A measurement of how hard the heart is contracting during ventricular systole.

3′ poly-A tail In eukaryotes, a series of adenine residues added to the 3′ end of an mRNA to enhance stability.

tagma A compound body section of an arthropod resulting from embryonic fusion of two or more segments.

TALE (transcription activator-like effectors) Proteins that can bind DNA in a sequence-specific fashion, used in DNA editing.

Taq polymerase A DNA polymerase isolated from Thermus aquaticus used in PCR amplification.

TATA box In eukaryotes, a sequence located upstream of the transcription start site.

taxis An orientation movement by a simple organism in response to an environmental stimulus.

taxonomy The science of classifying living things.

taxon Species or higher-level group, such as genus or family.

T cell A type of lymphocyte involved in cell-mediated immunity.

telencephalon The most anterior portion of the brain, including the cerebrum.

telomerase An enzyme that synthesizes telomeres on eukaryotic chromosomes.

telomere A specialized nontranscribed structure that caps each end of a chromosome.

telophase The phase of cell division where the spindle breaks down and nuclear envelopes form.

telson The tail spine of lobsters and crayfish.

temperate (lysogenic) phage A virus capable of incorporating its DNA into the host cell’s DNA.

template strand The DNA strand that is used as a template in transcription.

tendon A strap of cartilage that attaches muscle to bone.

tensile strength A measure of the cohesiveness of a substance; its resistance to being broken apart.

tertiary structure The folded shape of a protein produced by hydrophobic interactions and other forces.

testcross A mating between a dominant individual of unknown genotype and a homozygous tester.

testis In mammals, the sperm-producing organ.

tetanus Sustained forceful muscle contraction with no relaxation.

thalamus Part of the vertebrate forebrain that governs the flow of information to the cerebrum.

therapeutic cloning The use of somatic cell nuclear transfer to create stem cells.

thermodynamics The study of transformations of energy using heat as a form of measurement.

thermogenesis Generation of internal heat by endothermic animals to modulate temperature.

thigmomorphogenesis A developmental change in plants due to mechanical stress (e.g., wind).

thigmotropism In plants, unequal growth in some structure due to physical contact with an object.

threshold The minimum amount of stimulus required for a nerve to fire.

thylakoid A complex, organized internal membrane in chloroplasts containing photosystems.

Ti (tumor-inducing) plasmid A plasmid from Agrobacterium tumefaciens used to introduce recombinant DNA into plants.

tidal volume The amount of air inhaled and exhaled in one resting breath.

tight junction Region of actual fusion of plasma membranes between two adjacent animal cells.

tissue A group of similar cells organized into a structural and functional unit.

tissue plasminogen activator (TPA) A human protein that causes blood clots to dissolve.

tissue-specific stem cell A stem cell capable of developing into the cells of a certain tissue.

tissue system In plants, any of the three types of tissue (dermal, vascular, ground).

tissue tropism The affinity of a virus for certain cells within a multicellular host.

tonicity A measure of the tendency of a solution to change the volume of a cell by osmosis.

tonoplast The membrane surrounding the central vacuole in plant cells.

topoisomerase An enzyme that can change the topological state of DNA to relieve torsion.

torsion The process in gastropod development where the mantle cavity and anus move to the front of the body.

totipotent A cell that possesses the full genetic potential of the organism.

trachea A tube for breathing in terrestrial vertebrates and insects.

tracheids In plant xylem, dead cells that taper at the ends and overlap one another.

tracheole The smallest branches of the respiratory system of terrestrial arthropods.

trait In genetics, a characteristic that has alternative forms.

transcription The enzyme-catalyzed assembly of an RNA molecule complementary to a strand of DNA.

transcription bubble The region containing the RNA polymerase, DNA template, and RNA transcript.

transcription complex The complex of RNA polymerase II plus activators and factors engaged in transcribing DNA.

transcription factor Proteins required for RNA polymerase to bind to a eukaryotic promoter region.

transcription unit The region of DNA between a promoter and a terminator.

transcriptome All the RNA present in a cell or tissue at a given time.

transduction The process by which viruses can transfer genetic material from one host to another.

transfection The transformation of eukaryotic cells in culture.

transfer RNA (tRNA) A class of small RNAs that carry specific amino acids to the ribosome.

transformation The uptake of DNA directly from the environment.

transgenic organism An organism into which a gene has been introduced through genetic engineering.

translation The assembly of a protein on the ribosomes using mRNA to specify amino acids.

translation repressor protein A protein that prevents translation of mRNA by binding to the transcript.

translocation In plants, long-distance transport of food; in genetics, interchange of chromosome segments.

transmembrane domain Hydrophobic region of a transmembrane protein that anchors it in the membrane.

transmembrane route In plant roots, the pathway for movement of minerals that crosses cell membranes and vacuoles.

transpiration The loss of water vapor by plant parts through stomata.

transposable elements Segments of DNA able to move from one location on a chromosome to another.

transposition Type of genetic recombination in which transposable elements move randomly.

trichome In plants, a hairlike outgrowth from an epidermal cell.

triglyceride An individual fat molecule composed of a glycerol and three fatty acids.

triploid Possessing three sets of chromosomes.

trisomic Condition in which an additional chromosome has been gained due to nondisjunction.

trochophore A specialized type of free-living larva found in lophotrochozoans.

trophic level A step in the movement of energy through an ecosystem.

trophoblast In vertebrate embryos, the outer ectodermal layer of the blastodermic vesicle.

tropism Response to an external stimulus.

tropomyosin Low-molecular-weight protein surrounding the actin filaments of striated muscle.

troponin Complex of globular proteins that serves as a calcium-dependent switch in muscle contraction.

trp operon In E. coli, the operon containing genes for tryptophan synthesis.

true-breeding Breed of organism in which offspring are uniform and consistent.

tube foot In echinoderms, a flexible extension of the water–vascular system.

tubulin Globular protein subunit forming the hollow cylinder of microtubules.

tumor-suppressor gene A gene that normally functions to inhibit cell division.

turgor pressure The internal pressure inside a plant cell resulting from osmotic intake of water.

tympanum A thin membrane in some insects that functions as a sound receptor.

ubiquitin A 76-amino-acid protein that virtually all eukaryotic cells attach as a marker to proteins that are to be degraded.

unequal crossing over A process by which a crossover in a small region of misalignment at synapsis causes two homologous chromosomes to exchange segments of unequal length.

uniporter A carrier protein in a cell’s membrane that transports only a single type of molecule or ion.

uniramous Single-branched; describes the appendages of insects.

unsaturated fat A fat molecule in which one or more of the fatty acids contain fewer than the maximum number of hydrogens attached to their carbons.

urea An organic molecule formed in the vertebrate liver; the principal form of disposal of nitrogenous wastes by mammals.

urethra The tube carrying urine from the bladder to the exterior of mammals.

uric acid Insoluble nitrogenous waste products produced largely by reptiles, birds, and insects.

urine The liquid waste filtered from the blood by the kidney and stored in the bladder pending elimination through the urethra.

uropod One of a group of flattened appendages at the end of the abdomen of lobsters and crayfish that collectively act as a tail for a rapid burst of speed.

uterus In mammals, a chamber in which the developing embryo is contained and nurtured during pregnancy.

vacuole A membrane-bounded sac in the cytoplasm of some cells, used for storage or digestion purposes; plant cells often contain a large central vacuole.

valence electron An electron in the outermost energy level of an atom.

variable A factor that influences a process, outcome, or observation. In experiments, scientists attempt to isolate variables to test hypotheses.

vascular cambium In vascular plants, a cylindrical sheath of meristematic cells, the division of which produces secondary phloem outwardly and secondary xylem inwardly.

vascular tissue Containing or concerning vessels that conduct fluid.

vas deferens In mammals, the tube carrying sperm from the testes to the urethra.

vasopressin A posterior-pituitary hormone that regulates the kidney’s retention of water.

vector In molecular biology, a plasmid, phage, or artificial chromosome that allows propagation of recombinant DNA in a host cell into which it is introduced.

vegetal pole The hemisphere of the zygote comprising cells rich in yolk.

vein (1) In plants, a vascular bundle forming a part of the framework of the conducting and supporting tissue. (2) In animals, a blood vessel carrying blood from the tissues to the heart.

veliger The second larval stage of mollusks following the trochophore stage, during which the beginning of a foot, shell, and mantle can be seen.

ventricle A muscular chamber of the heart that receives blood from an atrium and pumps blood out to either the lungs or the body tissues.

vernalization A period of chilling required for the production of shoots or flowers in plants and seeds.

vertebrate A chordate with a spinal column; the notochord develops into the vertebral column composed of a series of vertebrae.

vertical gene transfer (VGT) The passing of genes from one generation to the next within a species.

vesicle A small intracellular, membrane-bounded sac in which various substances are transported or stored.

vessel element In vascular plants, a typically elongated cell, dead at maturity, which conducts water and solutes in the xylem.

vestibular apparatus The complicated sensory apparatus of the inner ear that provides for balance and orientation of the head in vertebrates.

vestigial structure A morphological feature that has no apparent current function and is thought to be an evolutionary relic.

villus In vertebrates, one of the minute, fingerlike projections lining the small intestine that serve to increase the absorptive surface area.

virion A single virus particle.

viroid Any of a group of small, naked RNA molecules that are capable of causing plant diseases, presumably by disrupting chromosome integrity.

virus Any of a group of complex biochemical entities consisting of genetic material wrapped in protein; viruses can reproduce only within living host cells.

visceral mass Internal organs in the body cavity of an animal.

vital capacity The maximum amount of air that can be inhaled and subsequently exhaled.

vitamin An organic substance that cannot be synthesized by a particular organism but is required in small amounts for normal metabolic function.

viviparity Refers to reproduction in which eggs develop within the mother’s body and young are born free-living.

voltage-gated ion channel A transmembrane pathway for an ion that is opened or closed by a change in the voltage, or charge difference, across the plasma membrane.

water potential The potential energy of water molecules. Regardless of the reason (e.g., gravity, pressure), water moves from a region where water potential is greater to a region where it is lower.

water–vascular system A fluid-filled hydraulic system found only in echinoderms that provides body support and a unique type of locomotion via tube feet.

Western blot A blotting technique used to identify specific protein sequences in a complex mixture. See Southern blot.

wild type In genetics, the phenotype or genotype that is characteristic of the majority of individuals of a species in a natural environment.

wobble pairing Refers to flexibility in the pairing between the base at the 5′ end of a tRNA anticodon and the base at the 3′ end of an mRNA codon.

wound response In plants, a signaling pathway initiated by leaf damage and leading to the production of proteinase inhibitors that give herbivores indigestion.

X-linked See sex-linked.

X chromosome One of two sex chromosomes; in mammals and in Drosophila, female individuals have two X chromosomes.

xylem In vascular plants, a specialized tissue, composed primarily of elongate, thick-walled conducting cells, which transports water and solutes through the plant body.

Y chromosome One of two sex chromosomes; in mammals and in Drosophila, male individuals have a Y chromosome and an X chromosome; the Y determines maleness.

yolk plug A plug occurring in the blastopore of amphibians during formation of the archenteron in embryological development.

yolk sac The membrane that surrounds the yolk of an egg and connects the yolk, a rich food supply, to the embryo via blood vessels.

zinc finger motif A type of DNA-binding motif in regulatory proteins that incorporates zinc atoms in its structure.

zona pellucida An outer membrane that encases a mammalian egg.

zone of cell division In plants, the part of the young root that includes the root apical meristem and the cells just posterior to it; cells in this zone divide every 12–36 hr.

zone of elongation In plants, the part of the young root that lies just posterior to the zone of cell division; cells in this zone elongate, causing the root to lengthen.

zone of maturation In plants, the part of the root that lies posterior to the zone of elongation; cells in this zone differentiate into specific cell types.

zone of polarizing activity Region in the developing vertebrate limb bud instrumental in proper growth of the digits along the anterior–posterior axis.

zoospore A motile spore.

zooxanthellae Symbiotic photosynthetic protists in the tissues of corals.

zygomycetes A type of fungus whose chief characteristic is the production of sexual structures called zygosporangia, which result from the fusion of two of its simple reproductive organs.

zygote The diploid (2n) cell resulting from the fusion of male and female gametes (fertilization).

Biology Glossary

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