Cell Cycle and Cell Division

2.1 Core concepts

• Cell cycle defined. The sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells is termed the cell cycle. Cell growth (cytoplasmic increase) is continuous, but DNA synthesis occurs only during one specific stage of the cell cycle; the replicated chromosomes are then distributed to daughter nuclei by a complex series of events that are under genetic control (NCERT §10.1, p. 120).
• Cycle duration. A typical eukaryotic cell cycle is illustrated by human cells in culture — they divide once approximately every 24 hours. Duration varies between organisms and cell types — yeast can progress through the cell cycle in only about 90 minutes (NCERT §10.1.1, p. 121).
• Two basic phases. The cell cycle is divided into Interphase and M Phase (Mitosis phase). M phase represents actual cell division (mitosis); interphase is between two successive M phases. In a 24-hour human cell cycle, cell division proper lasts only about an hour and the interphase lasts more than 95 per cent of the duration (NCERT §10.1.1, p. 121).
• M phase = karyokinesis + cytokinesis. M phase starts with nuclear division (karyokinesis) corresponding to separation of daughter chromosomes, and usually ends with division of cytoplasm (cytokinesis). Interphase, also called the "resting" phase, is when the cell prepares for division through cell growth and DNA replication (NCERT §10.1.1, p. 121).
• Sub-phases of interphase — G1, S, G2. G1 (Gap 1) corresponds to the interval between mitosis and initiation of DNA replication; the cell is metabolically active and continuously grows but does NOT replicate DNA. S phase (Synthesis) marks the period of DNA synthesis/replication — the DNA per cell doubles from 2C to 4C. However, there is no increase in chromosome number — if the cell had 2n chromosomes at G1, after S phase the number stays 2n (each chromosome now has two sister chromatids). G2 (Gap 2) — proteins are synthesised in preparation for mitosis while cell growth continues (NCERT §10.1.1, pp. 121–122).
• S-phase animal cell specifics. In animal cells, during the S phase, DNA replication begins in the nucleus and the centriole duplicates in the cytoplasm (NCERT §10.1.1, p. 121).
• Quiescent G0 stage. Some cells in the adult animals (e.g., heart cells) do not appear to exhibit division; many other cells divide only occasionally to replace cells lost by injury or cell death. Cells that do not divide further exit G1 to enter an inactive quiescent stage (G0) of the cell cycle — they remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism (NCERT §10.1.1, p. 122).
• Where mitosis happens. In animals mitotic cell division is only seen in diploid somatic cells. There are few exceptions to this where haploid cells divide by mitosis — for example, male honey bees. Plants can show mitotic divisions in both haploid and diploid cells (NCERT §10.1.1, p. 122).
• M phase — most dramatic period. Involves a major reorganisation of virtually all components of the cell; because chromosome number in parent and progeny is the same, it is also called equational division. Karyokinesis is divided for convenience into four stages: Prophase, Metaphase, Anaphase, Telophase (NCERT §10.2, p. 122).
• Prophase (mitotic). Follows S and G2 — the new DNA molecules are not distinct but intertwined. Prophase is marked by initiation of condensation of chromosomal material (chromatin untangles during condensation). Two characteristic events: (i) chromosomal material condenses to form compact mitotic chromosomes — each chromosome is seen to be composed of two chromatids attached together at the centromere; (ii) the centrosome, which underwent duplication during S phase, begins to move towards opposite poles; each centrosome radiates out microtubules called asters; the two asters together with the spindle fibres form the mitotic apparatus. Cells at the end of prophase, viewed under the microscope, do not show golgi complexes, ER, nucleolus and the nuclear envelope (NCERT §10.2.1, pp. 122–123).
• Metaphase. The complete disintegration of the nuclear envelope marks the start of metaphase. Chromosomes are spread through the cytoplasm; condensation is now complete and chromosomes are best studied morphologically at this stage. Each metaphase chromosome is made of two sister chromatids held together by the centromere. Small disc-shaped structures at the surface of the centromeres are called kinetochores, which serve as sites of attachment of spindle fibres. Metaphase plate is the plane of alignment of chromosomes at the equator — one chromatid of each chromosome is connected by its kinetochore to spindle fibres from one pole, its sister chromatid to fibres from the opposite pole. Key features: spindle fibres attach to kinetochores; chromosomes are aligned at the equatorial plate (NCERT §10.2.2, p. 123).
• Anaphase. At its onset, each chromosome arranged at the metaphase plate is split simultaneously and the two daughter chromatids — now referred to as daughter chromosomes — begin migration towards opposite poles. As each chromosome moves away from the equator, the centromere of each chromosome remains directed towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind. Key events: centromeres split and chromatids separate; chromatids move to opposite poles (NCERT §10.2.3, pp. 123–124).
• Telophase. Chromosomes that reach their respective poles decondense and lose their individuality; individual chromosomes can no longer be seen. Each set of chromatin material collects at each pole. Key events: chromosomes cluster at opposite spindle poles and lose identity as discrete elements; nuclear envelope develops around the chromosome clusters at each pole, forming two daughter nuclei; nucleolus, golgi complex and ER reform (NCERT §10.2.4, p. 124).
• Cytokinesis. Mitosis accomplishes not only segregation of duplicated chromosomes into daughter nuclei (karyokinesis) but also division of the cell itself by separation of cytoplasm — cytokinesis — at the end of which cell division is complete. In an animal cell, a furrow appears in the plasma membrane which gradually deepens and ultimately joins in the centre dividing the cell cytoplasm into two. Plant cells are enclosed by a relatively inextensible cell wall, so cytokinesis uses a different mechanism — wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls. The new wall begins with a simple precursor called the cell-plate that represents the middle lamella between the walls of two adjacent cells. Organelles (mitochondria, plastids) get distributed between the two daughter cells. In some organisms, karyokinesis is not followed by cytokinesis, producing a multinucleate condition — syncytium (e.g., liquid endosperm of coconut) (NCERT §10.2.5, p. 124).
• Significance of mitosis. Mitosis (equational division) is usually restricted to diploid cells, although in some lower plants and in some social insects haploid cells also divide by mitosis. Mitosis usually results in production of diploid daughter cells with identical genetic complement. Growth of multicellular organisms is due to mitosis. Cell growth disturbs the nucleo-cytoplasmic ratio, so the cell divides to restore it. A very significant contribution of mitosis is cell repair — cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are constantly replaced. Mitotic divisions in meristematic tissues — the apical and the lateral cambium — result in continuous growth of plants throughout their life (NCERT §10.3, p. 125).
• Meiosis — key features. Production of offspring by sexual reproduction includes fusion of two gametes, each with a complete haploid set of chromosomes. Gametes are formed from specialised diploid cells. This specialised division reduces the chromosome number by half — meiosis. Meiosis ensures the production of haploid phase in the life cycle of sexually reproducing organisms; fertilisation restores diploidy. Key features: (a) two sequential cycles of nuclear and cell division — meiosis I and meiosis II — but only a single cycle of DNA replication; (b) meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase; (c) pairing of homologous chromosomes and recombination between non-sister chromatids of homologous chromosomes; (d) four haploid cells are formed at the end of meiosis II (NCERT §10.4, p. 125).
• Prophase I. Typically longer and more complex than mitotic prophase; further subdivided into five phases — Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis — based on chromosomal behaviour. Leptotene — chromosomes become gradually visible under the light microscope; compaction continues throughout. Zygotene — chromosomes start pairing (synapsis); paired chromosomes are called homologous chromosomes; electron micrographs show that synapsis is accompanied by formation of a complex structure called the synaptonemal complex; the complex formed by a pair of synapsed homologous chromosomes is called a bivalent or tetrad. Pachytene — the four chromatids of each bivalent become distinct and clearly appear as tetrads; this stage is characterised by recombination nodules at which crossing over occurs between non-sister chromatids of homologous chromosomes. Crossing over is an enzyme-mediated process; the enzyme involved is called recombinase. Recombination between homologous chromosomes is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over. Diplotene — recognised by the dissolution of the synaptonemal complex and tendency of recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers — these X-shaped structures are called chiasmata. In oocytes of some vertebrates, diplotene can last for months or years. Diakinesis — marked by terminalisation of chiasmata; chromosomes are fully condensed; the meiotic spindle is assembled; by the end of diakinesis the nucleolus disappears and the nuclear envelope breaks down; diakinesis represents transition to metaphase (NCERT §10.4.1, p. 126).
• Metaphase I. The bivalent chromosomes align on the equatorial plate (Figure 10.3). The microtubules from opposite poles of the spindle attach to the kinetochores of homologous chromosomes (NCERT §10.4.1, pp. 126–127).
• Anaphase I. The homologous chromosomes separate, while sister chromatids remain associated at their centromeres — this is the reductional step (NCERT §10.4.1, p. 127).
• Telophase I. The nuclear membrane and nucleolus reappear; cytokinesis follows — called a dyad of cells. Chromosomes may undergo some dispersion but do not reach the extremely extended state of the interphase nucleus. The stage between the two meiotic divisions is called interkinesis; it is generally short-lived; there is no replication of DNA during interkinesis. Interkinesis is followed by prophase II, a much simpler prophase (NCERT §10.4.1, p. 127).
• Meiosis II — Prophase II. Initiated immediately after cytokinesis, usually before the chromosomes have fully elongated. In contrast to meiosis I, meiosis II resembles a normal mitosis. The nuclear membrane disappears by the end of prophase II; chromosomes again become compact (NCERT §10.4.2, p. 127).
• Metaphase II. Chromosomes align at the equator and microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids (NCERT §10.4.2, p. 127).
• Anaphase II. Begins with the simultaneous splitting of the centromere of each chromosome (which was holding the sister chromatids together), allowing them to move toward opposite poles by shortening of microtubules attached to kinetochores (NCERT §10.4.2, p. 127).
• Telophase II. Meiosis ends with telophase II — the two groups of chromosomes once again get enclosed by a nuclear envelope; cytokinesis follows, resulting in the formation of a tetrad of cells, i.e., four haploid daughter cells (NCERT §10.4.2, p. 128).
• Significance of meiosis. Meiosis is the mechanism by which conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms — even though it paradoxically reduces the chromosome number by half. It also increases genetic variability in the population from one generation to the next; variations are very important for the process of evolution (NCERT §10.5, p. 128).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Figure 10.1 (p. 121) — circular diagram of the cell cycle showing G1 → S → G2 within interphase and Prophase → Metaphase → Anaphase → Telophase → Cytokinesis within M phase. M is a tiny wedge; interphase fills >95% of the cycle.
• Figure 10.2 a–e (pp. 123–124) — stages of mitosis: Early Prophase (untangled threads), Late Prophase (a) (compact chromosomes with two sister chromatids + asters), Transition to Metaphase, Metaphase (b) (chromosomes at equator with kinetochore-attached spindle fibres from both poles), Anaphase (c) (chromatids moving to poles with centromere leading and arms trailing), Telophase (d) (decondensation at poles, nuclear envelope reforms), Interphase (e) (after cytokinesis).
• Figure 10.3 (p. 127) — meiosis I: Prophase I (bivalents with chiasmata visible), Metaphase I (bivalents at equator), Anaphase I (homologues separating with sister chromatids still attached), Telophase I (dyad).
• Figure 10.4 (p. 128) — meiosis II: Prophase II → Metaphase II → Anaphase II (sister chromatids separating after centromere split) → Telophase II (tetrad of four haploid cells).
• DNA/chromosome arithmetic (from §10.1.1, p. 121): if 2C at G1, then 4C after S = 4C at G2, drops to 2C after M in each daughter; chromosome number 2n stays 2n throughout interphase and after mitosis.
• Onion root tip recall (p. 122 margin box) — 16 chromosomes at G1, S, G2, after M; DNA goes 2C → 4C → 4C → 2C if M phase content is 2C.

2.4 Common confusions / NTA trap points

• "Reductional vs equational" — Meiosis I is reductional (homologues separate, 2n → n); Meiosis II is equational (sister chromatids separate, like mitosis). Don't say "meiosis = reductional" without qualifying which division.
• "Centromere splits in anaphase" — true for mitotic anaphase and anaphase II, NOT for anaphase I (where centromeres stay intact and only homologues separate).
• "Crossing over happens in zygotene" — WRONG. Synapsis is in zygotene; crossing over is in pachytene.
• "Chiasmata appear in pachytene" — WRONG; they appear in diplotene when the synaptonemal complex dissolves; they are terminalised in diakinesis.
• "DNA doubles in S, so chromosome number doubles" — WRONG. DNA goes 2C → 4C but chromosome number remains 2n (each chromosome now has two sister chromatids).
• "Cleavage furrow in plants" — WRONG. Plants use a cell-plate built outward from the centre; the furrow is animal-only because plants have a rigid cell wall.
• "Interkinesis has DNA replication" — WRONG. No DNA replication occurs between meiosis I and meiosis II.
• "G0 cells are dead/inactive" — WRONG. They are metabolically active; they just no longer proliferate.
• "Mitosis = only diploid cells everywhere" — WRONG. Honey bee males show haploid mitosis; plants show both haploid and diploid mitosis.
• "Nuclear envelope breaks down in prophase" — WRONG; it disintegrates at the start of metaphase per NCERT (golgi/ER/nucleolus/nuclear envelope are "not visible" at the end of prophase, but disintegration "marks the start of metaphase").
• "All recombination occurs at chiasmata in diakinesis" — WRONG; recombination is complete by the end of pachytene; diplotene's chiasmata are merely visible evidence.
• "Syncytium needs failure of karyokinesis" — WRONG. Syncytium arises when karyokinesis is NOT followed by cytokinesis (many nuclei + one cytoplasm).

2.5 Key processes / classifications