Chapter overview: cell cycle and division
All organisms begin as a single cell. Growth and reproduction depend on cell division: each parental cell divides to form two daughter cells, which can themselves grow and divide. The coordinated sequence in which a cell duplicates its genome, synthesises cellular constituents and divides is the cell cycle.
10.1 Cell cycle and interphase
A typical human cell in culture completes a cell cycle in about 24 hours, although duration varies with organism and cell type; yeast can complete one in about 90 minutes. The cell cycle has two basic phases: interphase and M phase. Interphase is the period between successive M phases and occupies more than 95% of a 24-hour human cell cycle. M phase is the actual division phase.
Interphase is not truly a resting period. The cell grows and replicates DNA in a tightly controlled order. It is divided into G1, S and G2 phases.
| Phase | NCERT events |
|---|---|
| G1 | Interval between mitosis and DNA replication; metabolically active cell grows continuously but does not replicate DNA. |
| S | DNA synthesis/replication; DNA content doubles from 2C to 4C, while chromosome number remains 2n. In animal cells, centrioles duplicate. |
| G2 | Cell growth continues and proteins are synthesised in preparation for mitosis. |
| G0 | Quiescent stage entered from G1; cells are metabolically active but do not proliferate unless required. |
10.2 M phase: mitosis and karyokinesis
M phase begins with nuclear division (karyokinesis) and generally ends with division of cytoplasm (cytokinesis). Mitosis conserves chromosome number between parent and progeny cells, so it is an equational division. Its nuclear events are progressive, conventionally grouped as prophase, metaphase, anaphase and telophase.
| Stage | Key events |
|---|---|
| Prophase | Chromatin condenses into chromosomes with two chromatids joined at centromere. Centrosomes move to opposite poles; asters and spindle fibres form. By end, Golgi, ER, nucleolus and nuclear envelope are not visible. |
| Metaphase | Chromosomes are maximally condensed and easiest to study. Spindle fibres attach to kinetochores; chromosomes align at the equator on metaphase plate. |
| Anaphase | Centromeres split; chromatids separate as daughter chromosomes and migrate to opposite poles, with centromeres leading. |
| Telophase | Chromosomes reach poles and decondense. Nuclear envelope, nucleolus, ER and Golgi reform around each chromosome set. |
Cytokinesis: animal cells and plant cells
After duplicated chromosomes segregate, cytokinesis divides the cell cytoplasm. In animal cells a cleavage furrow appears in the plasma membrane, deepens and joins at the centre. In plant cells, the inextensible wall prevents furrowing: wall formation starts at the centre as a cell plate and grows outward to meet lateral walls. The new plate forms the middle lamella between adjacent cells.
Mitochondria and plastids are distributed between daughter cells during cytoplasmic division. If karyokinesis is not followed by cytokinesis, a multinucleate syncytium forms; liquid endosperm in coconut is an example.
Significance of mitosis
Mitosis usually produces diploid daughter cells with identical genetic complement. It enables growth of multicellular organisms, restores the nucleo-cytoplasmic ratio when cells grow, and supports repair and replacement of cells such as epidermal, gut-lining and blood cells.
Mitotic divisions in apical and lateral meristems allow plants to grow throughout life. Mitosis is usually restricted to diploid somatic cells in animals, though haploid male honey bees are an exception. Plants may show mitosis in both haploid and diploid cells.
10.4 Meiosis: overview and key features
Meiosis occurs in specialised diploid cells that form gametes. It reduces chromosome number by half, producing haploid daughter cells; fertilisation restores the diploid phase. Meiosis has two sequential divisions, meiosis I and meiosis II, but only one DNA-replication cycle before meiosis I.
| Feature | Meiosis I | Meiosis II |
|---|---|---|
| Nature | Reductional division | Resembles mitosis; equational |
| Key separation | Homologous chromosomes separate | Sister chromatids separate |
| DNA replication beforehand | Occurs in preceding S phase | Absent; interkinesis has no DNA replication |
| Outcome | Two cells, often called a dyad | Four haploid cells, a tetrad |
Meiosis I: prophase I substages
Prophase I is longer and more complex than mitotic prophase. Its five substages are leptotene, zygotene, pachytene, diplotene and diakinesis.
| Substage | Events and terms |
|---|---|
| Leptotene | Chromosomes gradually become visible; compaction continues. |
| Zygotene | Homologous chromosomes pair by synapsis; the synaptonemal complex forms. A synapsed pair is a bivalent. |
| Pachytene | Four chromatids of each bivalent are distinct as a tetrad. Crossing over occurs between non-sister chromatids at recombination nodules, using recombinase. |
| Diplotene | Synaptonemal complex dissolves and homologues begin separating except at crossover sites, seen as chiasmata. In some vertebrate oocytes this can last months or years. |
| Diakinesis | Chiasmata terminalise, chromosomes fully condense and meiotic spindle assembles. Nucleolus disappears and nuclear envelope breaks down. |
Meiosis I and meiosis II stages
In metaphase I, bivalents align on the equatorial plate and spindle microtubules attach to homologous chromosomes. In anaphase I, homologous chromosomes separate but sister chromatids remain connected at centromeres. In telophase I, nuclear membrane and nucleolus may reappear and cytokinesis forms a dyad. Interkinesis is the short interval before meiosis II; no DNA replication occurs.
Meiosis II begins immediately after cytokinesis, usually before chromosomes fully elongate. It resembles mitosis: in prophase II the nuclear membrane disappears; in metaphase II chromosomes align at the equator; in anaphase II centromeres split and sister chromatids move apart; in telophase II nuclear envelopes reform and cytokinesis produces four haploid daughter cells.
Significance of meiosis and NCERT revision prompts
Meiosis conserves the species-specific chromosome number across sexual generations: gametes are haploid, and fertilisation restores diploidy. Pairing, crossing over and independent chromosome behaviour create genetic variation, which is essential for evolution.
- What is the average cell-cycle span of a mammalian cell?
- Differentiate karyokinesis and cytokinesis.
- Describe events in G1, S, G2 and G0.
- Why is mitosis called equational division?
- State the events in each stage of mitosis.
- Compare cytokinesis in plant and animal cells.
- Define synapsis, bivalent, tetrad and chiasmata.
- Compare anaphase of mitosis, anaphase I and anaphase II.
- List the significance of mitosis and meiosis.