THE LIVING WORLD

This chapter explains why DNA is the genetic material in most organisms, how it is packaged and copied, how genes direct RNA and protein synthesis, and how genomes are studied.

A nucleotide has a nitrogen base, pentose sugar and phosphate. DNA has deoxyribose and thymine; RNA has ribose and uracil. Watson and Crick described DNA as a double helix with antiparallel strands.

• Adjacent base pairs are 0.34 nm apart; one turn is 3.4 nm and contains 10 base pairs.
• Chargaff's rule: A = T and G = C. A-T has two hydrogen bonds; G-C has three.
• Griffith showed transformation; Avery, MacLeod and McCarty identified DNA as transforming principle; Hershey and Chase gave decisive phage evidence.
• RNA is genetic material in some viruses and can act as messenger, adapter, structural and catalytic molecule.

DNA is negatively charged and associates with positively charged histones in eukaryotes. A nucleosome contains about 146 bp of DNA around a histone octamer of two each of H2A, H2B, H3 and H4; H1 binds linker DNA.

• Euchromatin is loosely packed, lightly stained and transcriptionally active; heterochromatin is dense and inactive.
• Replication is semiconservative, as demonstrated by Meselson and Stahl using ¹⁵N-labelled E. coli.
• DNA polymerase synthesises only 5′ → 3′ and needs an RNA primer. Leading-strand synthesis is continuous; lagging-strand synthesis makes Okazaki fragments joined by ligase.

A transcription unit includes promoter, structural gene and terminator. RNA polymerase uses the template strand to make RNA 5′ → 3′; the coding strand has the same sequence as RNA except that DNA has T instead of U.

• Pol I makes 28S, 18S and 5.8S rRNAs; Pol II makes hnRNA; Pol III makes tRNA, 5S rRNA and snRNA.
• hnRNA becomes mRNA by 5′ capping, 3′ tailing and splicing. Introns are removed and exons joined.
• tRNA is the adaptor molecule with an anticodon loop and 3′ amino-acid acceptor end; its 3D form is inverted L.

The genetic code is triplet, nearly universal, non-overlapping, commaless, unambiguous and degenerate. Of 64 codons, 61 specify amino acids; UAA, UAG and UGA are stop codons. AUG initiates translation and codes for methionine.

Ribosomes read mRNA codons and tRNA anticodons deliver amino acids. Multiple ribosomes on one mRNA form a polysome.

Jacob and Monod described the inducible lac operon of E. coli. Regulator gene i produces a repressor; structural genes z, y and a produce enzymes for lactose metabolism.

• Without lactose, active repressor binds operator and transcription is off.
• With lactose, inducer inactivates repressor and RNA polymerase transcribes the structural genes.

The Human Genome Project aimed to identify human genes, sequence about three billion bases, store information in databases and address ethical, legal and social issues.

• Human genome: about 3164.7 million bases; less than 2% codes for proteins; average gene about 3000 bases; 99.9% of bases identical among humans.
• DNA fingerprinting uses highly polymorphic repetitive DNA, particularly VNTR minisatellites.
• Alec Jeffreys developed the technique for forensics, paternity testing and identification; identical twins share a pattern.

• DNA structure and complementary base pairing enable stable information storage and copying.
• Replication is semiconservative and new DNA is made only 5′ → 3′.
• Transcription produces RNA and translation produces proteins through the genetic code.
• The lac operon illustrates gene regulation.
• Genome sequencing and DNA fingerprinting analyse genetic variation.