In this chapter: phases of the cell cycle, G0, mitosis and cytokinesis, significance of mitosis, meiosis I (with the five substages of prophase I) and meiosis II, significance of meiosis.The cell cycle
The cell cycle is the sequence of events by which a cell duplicates its genome, synthesises other constituents and divides into two daughter cells. A typical human cell in culture divides about once every 24 hours; yeast takes about 90 minutes.
- Interphase ("resting phase", though it is very active) lasts more than 95% of the cycle. Of a 24-hour human cycle, the M phase takes only about an hour.
- M phase (mitosis): karyokinesis (division of the nucleus) followed by cytokinesis (division of the cytoplasm).
| Phase | What happens |
|---|---|
| G1 (gap 1) | Cell is metabolically active and grows, but does not replicate DNA |
| S (synthesis) | DNA replication. DNA per cell doubles (2C to 4C) but the chromosome number stays the same (2n stays 2n). In animal cells, the centriole duplicates in the cytoplasm |
| G2 (gap 2) | Proteins for mitosis are synthesised; growth continues |
| G0 (quiescent) | Cells that exit G1 and stop dividing, e.g. heart cells. They remain metabolically active and can divide again when needed |
In animals, mitosis occurs only in diploid somatic cells. In plants, both haploid and diploid cells can divide by mitosis.
Mitosis (equational division)
| Stage | Key events |
|---|---|
| Prophase | Chromatin condenses into chromosomes (each with two chromatids joined at the centromere). Centrosomes move towards opposite poles and start forming spindle fibres. By the end, the Golgi, ER, nucleolus and nuclear envelope disappear |
| Metaphase | Nuclear envelope has fully disintegrated. Chromosomes are most condensed, so their morphology is best studied now. Spindle fibres attach to kinetochores (small disc-shaped structures at the centromere). Chromosomes line up at the metaphase plate |
| Anaphase | Centromeres split; sister chromatids separate and become daughter chromosomes, moving to opposite poles, centromere first, arms trailing |
| Telophase | Chromosomes reach the poles and decondense; nuclear envelope reassembles; nucleolus, Golgi and ER reform |
Cytokinesis
- Animal cells: a furrow appears in the plasma membrane and deepens until the cell splits in two.
- Plant cells: the rigid wall prevents furrowing. A cell plate forms in the centre and grows outwards to meet the lateral walls; it becomes the middle lamella.
- When karyokinesis is not followed by cytokinesis, a multinucleate condition (syncytium) forms, e.g. the liquid endosperm of coconut.
Significance of mitosis
Mitosis produces diploid daughter cells with identical genetic material; restores the nucleo-cytoplasmic ratio in growing cells; repairs tissue (the upper layer of the epidermis, the lining of the gut and blood cells are constantly replaced); and drives continuous growth at the apical and lateral meristems of plants.
Meiosis (reduction division)
Meiosis occurs during gamete formation. Its key features are two sequential divisions (meiosis I and II) with only one round of DNA replication, pairing of homologous chromosomes and recombination between them, and four haploid cells at the end.
Prophase I: five substages
| Substage | Key event |
|---|---|
| Leptotene | Chromosomes become gradually visible under the light microscope; compaction continues |
| Zygotene | Homologous chromosomes pair: synapsis, accompanied by the synaptonemal complex. A pair of synapsed homologues is a bivalent or tetrad |
| Pachytene | The four chromatids of each bivalent become clearly visible. Recombination nodules appear, the sites of crossing over between non-sister chromatids of homologous chromosomes, catalysed by the enzyme recombinase. Crossing over is completed by the end of pachytene |
| Diplotene | The synaptonemal complex dissolves; homologues begin to separate except at the sites of crossing over, which appear as X-shaped chiasmata. In the oocytes of some vertebrates, diplotene can last for months or years |
| Diakinesis | Terminalisation of chiasmata; chromosomes fully condensed; spindle assembles; nucleolus disappears and the nuclear envelope breaks down |
Memory line: Lazy Zebras Play Dirty Dice = Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis.
Rest of meiosis I
- Metaphase I: bivalents align on the equatorial plate; spindle fibres from opposite poles attach to the kinetochores of homologous chromosomes.
- Anaphase I: homologous chromosomes separate; sister chromatids stay together at their centromeres.
- Telophase I: nuclear membrane and nucleolus reappear; cytokinesis gives a dyad of cells.
- Interkinesis: generally short-lived; no DNA replication.
Meiosis II (like mitosis)
- Prophase II: chromosomes compact again; the nuclear membrane disappears by the end.
- Metaphase II: chromosomes align at the equator; spindle fibres attach to kinetochores of sister chromatids.
- Anaphase II: the centromere of each chromosome splits; sister chromatids move to opposite poles.
- Telophase II: nuclear envelopes form; cytokinesis gives a tetrad of four haploid cells.
Significance of meiosis
Meiosis keeps the chromosome number constant from generation to generation in sexually reproducing organisms (even though gametes are haploid, fertilisation restores diploidy), and increases genetic variability through crossing over and independent assortment. Variation is important for evolution.
Mitosis vs meiosis
| Mitosis | Meiosis | |
|---|---|---|
| Where | Somatic cells | Reproductive (gamete-forming) cells |
| Divisions | One | Two, with one DNA replication |
| Daughter cells | Two, diploid, identical | Four, haploid, genetically different |
| Pairing / crossing over | No | Yes (zygotene / pachytene) |
| What separates in anaphase | Sister chromatids | Homologous chromosomes (anaphase I); sister chromatids (anaphase II) |
Common traps: (1) In S phase, DNA content doubles but chromosome number does not. (2) Crossing over happens in pachytene; chiasmata become visible in diplotene; terminalisation happens in diakinesis. (3) No DNA replication in interkinesis. (4) Chromosome morphology is best studied at metaphase.NEET focus
- Phases of the cycle and their events; G0; DNA content vs chromosome number (C vs n).
- Events of each mitotic stage; cell plate in plants; syncytium in coconut.
- Prophase I substages in order, with synaptonemal complex, recombinase and chiasmata.
- Anaphase I vs anaphase II; significance of both divisions.
Practice questions
If a cell has 2C DNA in G1, its DNA content at G2 and its chromosome number (2n at G1) will be:
- 4C, 4n
- 4C, 2n
- 2C, 2n
- 2C, 4n
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Crossing over occurs during:
- Leptotene
- Zygotene
- Pachytene
- Diakinesis
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The synaptonemal complex dissolves during:
- Zygotene
- Pachytene
- Diplotene
- Metaphase I
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Chromosome morphology is best studied at:
- Prophase
- Metaphase
- Anaphase
- Telophase
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The liquid endosperm of coconut is an example of:
- Cytokinesis without karyokinesis
- Karyokinesis without cytokinesis
- Meiosis
- Cell plate formation
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Homologous chromosomes separate during:
- Anaphase of mitosis
- Anaphase I
- Anaphase II
- Telophase II
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Cells that exit G1 and enter an inactive stage, such as heart cells, are in:
- S phase
- G2
- G0
- M phase