Cell Cycle and Cell Division — NEET Notes

The cell cycle — what it is

The cell cycle is the ordered sequence of events by which a cell duplicates its genome, synthesises the rest of its constituents, and divides into two daughter cells. Cell division, DNA replication, and cell growth all happen here, and they have to happen in a coordinated way — otherwise the daughter cells would not receive intact, complete genomes. The whole programme is under genetic control.

A typical human cell completes one cycle in roughly 24 hours, of which actual mitosis (the M phase) takes only about an hour. The remaining 95% is interphase — once misleadingly called the "resting phase," even though the cell is anything but resting. Yeast can finish the whole cycle in 90 minutes; different cell types in the same organism move at very different speeds.

At the highest level the cycle is split into two parts: the interphase, when the cell prepares for division, and the M phase, when nuclear division (karyokinesis) and cytoplasmic division (cytokinesis) actually occur. Interphase itself is divided into three subphases — G1, S, and G2.

Interphase — the four phases of the cell cycle

Each phase has its own job. Confusing them is the single most common NEET trap in this chapter, and the 2021, 2023, and 2020 papers all asked direct questions on what happens where.

The order is fixed: G1 → S → G2 → M, then back to G1 in the daughter cells. NEET 2019 asked this sequence in plain words. Note the bookkeeping carefully: at the end of G1 the cell is 2n with 2C of DNA; after S it is still 2n but now 4C; throughout G2 and into M it remains 2n with 4C; after M, each daughter cell is 2n with 2C again.

The quiescent stage (G0)

Not every cell stays on the cycle. Some cells in adult animals — heart muscle cells are the classic example — do not divide at all. Many other cells divide only occasionally, when called on to replace cells lost to injury or normal turnover. These cells exit G1 at a checkpoint and enter an inactive state called G0, or the quiescent stage. They remain metabolically active but stop proliferating until a signal pushes them back into G1.

NEET 2019 (Q.45) tested the language directly: "A cell in G0 phase is said to exit the cell cycle." NEET 2020 (Q.90) tested the position: G0 is entered at the end of G1, not S or G2.

M phase — the actual division

The M phase is "the most dramatic period of the cell cycle, involving a major reorganisation of virtually all components of the cell" — NCERT's own words. It is split into karyokinesis (nuclear division) followed by cytokinesis (cytoplasmic division). Karyokinesis itself is conventionally divided into four stages — prophase, metaphase, anaphase, telophase — though in reality the transitions are gradual.

Because the chromosome number of the parent and the two daughter cells is identical, mitosis is also called equational division. In animals, mitotic division is restricted to diploid somatic cells (with rare exceptions like haploid honey-bee males). In plants, both haploid and diploid cells can divide by mitosis — recall the alternation of generations from your reproductive-biology chapter.

Prophase to telophase — the four stages

Each stage has a small set of definitional events. Memorise these as a flow: condensation → equator → split → reform.

Two terminology traps NEET uses repeatedly:

• Spindle fibres attach to the kinetochore, not directly to the centromere. The kinetochore is the disc on the centromere. NEET 2022 (Q.167) marked "Spindle fibres attach to centromere of chromosomes" as incorrect.
• Synapsis — the pairing of homologous chromosomes — does not happen in mitosis. NEET 2016 (Q.64) and 2022 (Q.125) both tested this.

Cytokinesis — plant vs animal

Karyokinesis splits the nucleus; cytokinesis splits the cell. Animal and plant cells solve this problem differently because plants have a rigid cell wall to deal with.

NEET 2016 (Q.59) used a striking application: a student observed a plant cell at telophase in which no cell plate had formed, so the cell ended up with double the chromosomes of its neighbours. The correct outcome is polyploidy — exactly the failure mode of plant cytokinesis. Note also that when karyokinesis occurs without cytokinesis at all, you get a syncytium, the multinucleate condition seen famously in liquid coconut endosperm.

Significance of mitosis

Three functions are worth committing to memory:

• Growth. Multicellular organisms grow by adding cells, not by enlarging existing ones indefinitely. Mitotic divisions in apical and lateral meristems drive plant growth throughout life.
• Repair and replacement. The epidermis, gut lining, and blood-forming tissues lose cells constantly. Mitosis replaces them. The nucleo-cytoplasmic ratio is also restored — as a cell grows the cytoplasm outpaces the nucleus, so the cell must divide to bring the ratio back into balance.
• Maintenance of the diploid number. Because mitosis is equational, every somatic cell ends up with the species' chromosome number identical to the zygote it came from. No reduction, no variation — that genetic identity is the whole point.

Meiosis — overview

Sexually reproducing organisms must keep their chromosome number constant across generations. If fertilisation doubles the chromosome number — and it does — then somewhere along the line the chromosome number has to be halved. That halving is the job of meiosis: a specialised division in which a diploid cell produces four haploid daughter cells. In animals, meiosis occurs during gametogenesis (in the gonads). In plants, it occurs in specialised cells inside the sporogenous tissue of anthers and ovules.

Four features define meiosis, and NEET tests all four:

• Meiosis involves two sequential cycles of nuclear and cell division — meiosis I and meiosis II — but only a single cycle of DNA replication. NEET 2022 (Q.180) made this the answer: "DNA replication occurs in S phase of Meiosis II" is the incorrect statement.
• Meiosis I is initiated only after the parental chromosomes have replicated in S phase to produce identical sister chromatids.
• Meiosis involves pairing of homologous chromosomes (synapsis) and recombination between non-sister chromatids of homologous chromosomes.
• Four haploid cells are formed at the end of meiosis II.

The stage between meiosis I and meiosis II is called interkinesis. It is short, and crucially no DNA replication takes place during interkinesis.

Prophase I — the five sub-stages

Prophase of meiosis I is longer and far more complex than prophase of mitosis. It is sub-divided into five named stages based on chromosomal behaviour: Leptotene → Zygotene → Pachytene → Diplotene → Diakinesis. Memorise the order — at least three NEET PYQs from 2016–2023 simply ask "at which stage does X happen?"

Crossing over leads to recombination of genetic material on the two chromosomes — it is the molecular source of every shuffled-card combination an organism can produce.

Why pachytene matters — variation begins here

Crossing over is the central event of meiosis. Without it, sex would just be a reshuffling of intact parental chromosomes — useful, but limited. With it, the chromosomes themselves get recombined: a single homologue can carry genes from both grandparents. This is what makes meiosis the engine of biological variation, and what underwrites the entire science of genetics that you will read in Principles of Inheritance and Variation.

Metaphase I → telophase I

After diakinesis the bivalents move to the equator. From here on, meiosis I diverges sharply from mitosis in one critical respect: it is the homologous chromosomes, not the sister chromatids, that get separated.

• Metaphase I. The bivalents (not individual chromosomes) align on the equatorial plate. Spindle microtubules from opposite poles attach to the kinetochores of homologous chromosomes.
• Anaphase I. Homologous chromosomes separate and move to opposite poles, while sister chromatids stay joined at their centromeres. This is the reductional event — chromosome number is halved here.
• Telophase I. Nuclear membrane and nucleolus reappear, cytokinesis follows. The result is a dyad of cells — two cells, each n, each chromosome still made of two sister chromatids. Chromosomes typically do not fully decondense.

The stage between meiosis I and II is called interkinesis. It is short-lived, and — repeat after NCERT — no DNA replication occurs in interkinesis.

Meiosis II — like mitosis, but on haploid cells

Meiosis II resembles a normal mitosis, but it operates on haploid cells whose chromosomes still consist of two sister chromatids. Its purpose is to separate those sister chromatids, so that each of the four final cells carries one chromatid per chromosome.

Significance of meiosis

Meiosis does two jobs at once, and they pull in opposite directions:

• Conservation of chromosome number. By halving 2n to n, meiosis ensures that fertilisation — which doubles whatever it gets — restores the species' diploid number. Without this halving, every generation would double its chromosome count and the species would collapse within a few generations. This is the paradox: meiosis conserves the species number by reducing it.
• Generation of variation. Two independent mechanisms generate genetic variability — crossing over in pachytene, and independent assortment of maternal and paternal homologues at metaphase I. The combinatorial output is vast: a human cell with 23 chromosome pairs can generate 2²³ ≈ 8 million different gamete chromosome combinations from assortment alone, before counting recombination.

Variation is the raw material of evolution. Every adaptation Darwin described, every breed of dog, every drought-tolerant cereal in Indian agriculture — all of it begins as a recombination event in some ancestor's pachytene nucleus.

Mitosis vs meiosis — the comparison NEET wants

One more vocabulary set — chromosome morphology

NEET 2021 (Q.119) and 2018 (Q.148) leaned on a related vocabulary: the classification of chromosomes by centromere position. You will see this on any meiotic chromosome you draw, so it is worth a quick line each.

• Metacentric — centromere at the middle, two equal arms.
• Sub-metacentric — centromere slightly off-centre (L-shaped).
• Acrocentric — centromere very close to one end.
• Telocentric — centromere at the terminal end.
• Lampbrush chromosomes are diplotene bivalents of amphibian oocytes — heavily transcribed. Polytene chromosomes belong to insect (e.g., Chironomus) salivary glands, not amphibian oocytes — the NEET 2018 trap.