Cell Cycle and Cell Division
How a single cell grows and divides — by mitosis to make genetically identical body cells and by meiosis to halve chromosomes and make varied gametes — through the regulated stages of the cell cycle.
Science & Technology is a recurring Prelims zone, and NCERT cell biology supplies direct factual MCQs — stage-event matching, C-value vs ploidy, and structures/enzymes like recombinase, kinetochore and the synaptonemal complex. In GS-III Mains it is the base for biotechnology, cancer (uncontrolled cell division), stem cells and genetic-engineering debates, and for evaluating health/biotech current affairs.
Understand the chapter
Cell Cycle: Two Basic Phases
The cell cycle is the ordered sequence by which a cell duplicates its genome, synthesises its other constituents and divides into two daughter cells. It has two basic phases — the long Interphase (growth + DNA replication) and the short M Phase (actual division). Cytoplasmic growth is continuous, but DNA synthesis is confined to one specific stage, and all these events are under genetic control.
- Human cultured cells divide about once every 24 hours; yeast completes the cycle in about 90 minutes.
- Interphase occupies more than 95% of the cycle; M phase (division proper) lasts only about 1 hour in human cells.
- M phase = karyokinesis (nuclear division) + cytokinesis (cytoplasm division).
Interphase: G1, S, G2 and the Quiescent G0
Interphase, though misleadingly called the 'resting phase', is the most metabolically active preparatory period, split into G1, S and G2. G1 is the gap between mitosis and DNA replication, where the cell grows but does not replicate DNA; the S phase is when DNA replicates; G2 is when proteins are synthesised for mitosis while growth continues. Cells that do not divide (e.g., heart cells) exit G1 into an inactive quiescent stage called G0.
- S phase: DNA content doubles 2C -> 4C, but chromosome number is unchanged (2n stays 2n).
- In animal cells the centriole also duplicates in the cytoplasm during S phase.
- G0 (quiescent) cells stay metabolically active but stop proliferating unless required.
- Animals: mitosis only in diploid somatic cells (exception: haploid male honey bees); plants: mitosis in both haploid and diploid cells.
Mitosis: Karyokinesis in Four Stages (PMAT)
Mitosis is the equational division — parent and progeny cells carry the same chromosome number. Karyokinesis is a continuous process artificially divided into Prophase, Metaphase, Anaphase and Telophase. In Prophase, chromatin condenses into compact chromosomes (two chromatids joined at the centromere) and the centrosome (duplicated in S phase) moves to opposite poles, forming the mitotic apparatus of asters plus spindle fibres.
- Metaphase: nuclear envelope disintegrates; chromosomes align at the metaphase plate; spindle fibres attach at kinetochores — best stage to study chromosome morphology.
- Anaphase: centromeres split, sister chromatids separate and move to opposite poles (centromere leads, arms trail).
- Telophase: chromosomes decondense; nuclear envelope, nucleolus, golgi and ER reform, giving two daughter nuclei.
Cytokinesis and the Significance of Mitosis
After nuclear division, the cytoplasm divides by cytokinesis. Animal cells pinch in via a cleavage furrow in the plasma membrane; plant cells, bound by a rigid inextensible wall, build a cell-plate from the centre outward, representing the future middle lamella. Mitosis sustains growth, repair and the nucleo-cytoplasmic ratio.
- Syncytium = karyokinesis without cytokinesis, giving a multinucleate cell (e.g., liquid endosperm of coconut).
- Significance: produces genetically identical diploid cells and drives growth of multicellular organisms.
- Cell repair: epidermis, gut lining and blood cells are constantly replaced.
- Meristems (apical and lateral cambium) keep plants growing throughout life.
Meiosis: The Reductional Division
Meiosis halves the chromosome number, converting specialised diploid cells into haploid gametes during gametogenesis; fertilisation later restores diploidy. Its defining feature is two sequential nuclear and cell divisions (Meiosis I and II) with only a single round of DNA replication, ultimately producing four haploid cells. It also pairs homologous chromosomes and recombines them, generating variation.
- Meiosis I is reductional (halves the number); Meiosis II is equational and resembles a normal mitosis.
- Four haploid daughter cells result at the end of Meiosis II.
- Interkinesis, the short gap between the two divisions, has NO DNA replication.
Prophase I: Five Sub-stages of Recombination
Prophase I is far longer and more complex than mitotic prophase, subdivided into Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis. Pairing of homologues (synapsis) and the enzyme-mediated exchange of genetic material (crossing over) are its hallmark events. The remainder of Meiosis I separates homologous chromosomes, while Meiosis II separates sister chromatids.
- Zygotene: synapsis forms the synaptonemal complex; the paired homologues are a bivalent (tetrad).
- Pachytene: crossing over between non-sister chromatids via the enzyme recombinase, at recombination nodules.
- Diplotene: synaptonemal complex dissolves and X-shaped chiasmata appear (diplotene can last months/years in some vertebrate oocytes).
- Anaphase I: homologues separate but sister chromatids stay joined at their centromeres (centromeres do NOT split).
Key terms
- Cell cycle
- Ordered sequence by which a cell duplicates its genome, synthesises its constituents and divides into two daughter cells.
- Quiescent stage (G0)
- Inactive stage entered from G1 by non-dividing cells; metabolically active but no longer proliferating.
- Karyokinesis
- Division of the nucleus, i.e., separation of daughter chromosomes.
- Cytokinesis
- Division of the cytoplasm that completes cell division after karyokinesis.
- Kinetochore
- Disc-shaped structure at the surface of the centromere serving as the attachment site for spindle fibres.
- Synapsis
- Pairing of homologous chromosomes during zygotene of Prophase I.
- Synaptonemal complex
- Protein structure formed between synapsed homologous chromosomes during zygotene.
- Bivalent (tetrad)
- Complex of a pair of synapsed homologous chromosomes, containing four chromatids.
- Crossing over
- Recombinase-mediated exchange of genetic material between non-sister chromatids of homologous chromosomes (in pachytene).
- Chiasmata
- X-shaped sites where recombined homologues remain linked, visible in diplotene.
Must-know facts exam-ready
- Human cells in culture divide about every 24 hours; yeast completes the cell cycle in about 90 minutes.
- Interphase is more than 95% of the cycle; M phase (division proper) lasts only about 1 hour in human cells.
- Interphase = G1 + S + G2; DNA synthesis/replication occurs ONLY in the S phase.
- In S phase DNA doubles 2C -> 4C, but chromosome number stays 2n; in animal cells the centriole also duplicates.
- Mitosis is the equational division (same chromosome number in parent and progeny), mostly in diploid somatic cells.
- Karyokinesis order: Prophase -> Metaphase -> Anaphase -> Telophase.
- Asters + spindle fibres = mitotic apparatus; kinetochores at the centromere are the spindle-attachment sites.
- Animal cytokinesis = cleavage furrow; plant cytokinesis = cell-plate (middle lamella) forming centre-outward.
- Syncytium (karyokinesis without cytokinesis) example: liquid endosperm of coconut.
- Meiosis = one round of DNA replication + two divisions -> four haploid cells; interkinesis has no DNA replication.
- Prophase I order: Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis.
- Crossing over occurs in pachytene via recombinase; chiasmata appear in diplotene; exception — male honey bees (haploid) divide by mitosis.
Memory tricks remember it for good
Traps to avoid
- Mitosis is equational (2n -> 2n); the reduction happens at Meiosis I — Meiosis II is equational and resembles mitosis.
- In S phase DNA content doubles (2C -> 4C) but chromosome NUMBER stays 2n; aspirants wrongly turn 2n into 4n.
- Synapsis/pairing is in zygotene, crossing over in pachytene, and chiasmata become visible in diplotene — these sub-stages are frequently swapped.
- In Anaphase I centromeres do NOT split (homologues separate); centromere splitting/sister-chromatid separation is in mitotic Anaphase and Anaphase II.
- Interphase, though called the 'resting phase', is the longest and most active prep phase (>95%); G0 cells are metabolically active, just non-dividing.
- The synaptonemal complex forms in zygotene and dissolves in diplotene — not in pachytene.
Exam focus
🧠 Prelims angles
- Match-the-stage questions on mitosis/meiosis events (nuclear envelope breakdown, centromere splitting, alignment at plate).
- Sub-stages of Prophase I and the signature event of each (synapsis, crossing over, chiasmata, terminalisation).
- DNA content (C-value) vs ploidy (n) at G1, after S and at G2 — calculation-type MCQs.
- Key structures/enzyme: recombinase, synaptonemal complex, bivalent/tetrad, kinetochore, recombination nodule.
- Cell-cycle durations and proportions (human ~24 hr, yeast ~90 min; interphase >95%, M phase ~1 hr).
- Exceptions and examples: haploid mitosis in male honey bees; syncytium = coconut liquid endosperm.
✍️ Mains angles GS-III
- How does regulation (and loss of control) of the cell cycle explain cancer, and why does this make cell-cycle biology central to modern biotechnology?Link uncontrolled mitosis to tumour formation; connect to S&T applications like targeted therapies and stem-cell research.
- Explain how meiosis generates genetic variation and why this is significant for evolution and crop/animal breeding.Build on crossing over (recombination) plus independent assortment of homologues as the raw material for natural selection and improved varieties.
Last-minute revision tick as you recall
- Cell cycle = Interphase (G1-S-G2) + M phase; interphase >95%, human ~24 hr, yeast ~90 min.
- S phase: DNA doubles 2C -> 4C, chromosome number unchanged (2n); centriole duplicates in animal cells.
- G0 = quiescent, metabolically active but non-dividing (e.g., heart cells).
- Mitosis = equational (2n -> 2n); stages PMAT + cytokinesis; asters + spindle = mitotic apparatus.
- Anaphase: centromeres split, chromatids move to poles.
- Plant cytokinesis = cell-plate; animal = cleavage furrow; syncytium = coconut liquid endosperm.
- Meiosis = reductional; 1 DNA replication, 2 divisions -> 4 haploid cells; interkinesis has no replication.
- Prophase I: Leptotene -> Zygotene (synapsis) -> Pachytene (crossing over, recombinase) -> Diplotene (chiasmata) -> Diakinesis (terminalisation).
- Anaphase I: homologues separate, sisters stay joined; Meiosis II resembles mitosis.
Distilled from NCERT Class 11 · Biology (Class 11) for UPSC. Always cross-check facts with the original NCERT.