NEET Biology — Chapter 10

Cell Cycle and Cell Division

Cell Cycle and Cell Division covers the phases of the cell cycle (G₁, S, G₂, M), the events of mitosis (with each phase), and meiosis — with particular focus on Prophase I substages and the significance of crossing over. NEET sets 2–4 MCQs from this chapter every year. The five substages of Prophase I, the mitosis vs meiosis comparison table, and the timing of crossing over are the highest-yield topics.

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1. Cell Cycle — Phases and Duration

The cell cycle is the sequence of events from one cell division to the next. It has two major stages:

Interphase (~90–95% of cycle time) — cell grows and prepares for division:
— G₁ phase (first growth phase): cell grows; proteins synthesised; organelles increase.
— S phase (synthesis phase): DNA replication; each chromosome becomes a pair of sister chromatids.
— G₂ phase (second growth phase): cell continues to grow; prepares for mitosis.
— G₀ phase: quiescent; cells that have left the cycle (e.g., neurons, muscle cells).
M phase (Mitotic phase) — cell division: karyokinesis (nuclear division) + cytokinesis (cytoplasmic division).

NEET tip: DNA replication occurs only once per cell cycle, during S phase. G₁ and G₂ are checkpoints where the cell decides whether to proceed or not. Cyclins and CDKs (cyclin-dependent kinases) control progression through checkpoints.

2. Mitosis — Phases and Events

Mitosis produces two daughter cells genetically identical to the parent. It is the basis of growth, repair, and asexual reproduction. Mitosis has four stages plus cytokinesis:

Phase	Key events
Prophase	Chromatin condenses into chromosomes; spindle fibres begin to form; nucleolus disappears; nuclear envelope starts to break down.
Metaphase	Chromosomes align at the metaphase plate (equatorial plane); spindle fibres attach to centromeres. Best stage to count chromosomes.
Anaphase	Centromeres split; sister chromatids pulled to opposite poles by spindle. Number of chromosomes doubles transiently.
Telophase	Nuclear envelope reforms around each set; chromosomes decondense; nucleolus reappears.
Cytokinesis	Animal cells: cleavage furrow (actin ring). Plant cells: cell plate formed by Golgi vesicles.

3. Meiosis — Overview and Significance

Meiosis produces four haploid daughter cells from one diploid parent cell. It occurs in reproductive organs during gamete formation. Meiosis has two sequential divisions:

Meiosis I (reductional division) — homologous chromosomes separate → chromosome number halved.
Meiosis II (equational division) — sister chromatids separate → similar to mitosis.

Significance:

Maintains chromosome number across generations in sexually reproducing organisms.
Introduces genetic variation via crossing over and independent assortment.

NEET tip: Meiosis I is reductional; Meiosis II is equational (like mitosis). At the END of Meiosis I — cells are haploid but each chromosome still has two sister chromatids. DNA content and chromosome number become truly haploid only after Meiosis II.

4. Meiosis I — Prophase I in Detail

Prophase I is the longest and most complex phase of meiosis. It is divided into five substages (LZPDDIA):

Substage	Key events
Leptotene	Chromosomes begin to condense; become visible as thin threads.
Zygotene	Synapsis — homologous chromosomes pair up; synaptonemal complex forms; the paired structure = bivalent (tetrad).
Pachytene	Crossing over — exchange of segments between non-sister chromatids of homologous chromosomes at chiasmata; recombination nodules visible.
Diplotene	Synaptonemal complex dissolves; homologs remain attached at chiasmata; terminalisation of chiasmata begins.
Diakinesis	Chromosomes maximally condensed; nuclear envelope breaks; spindle forms; chiasmata move to chromosome ends.

NEET caution: Crossing over occurs during Pachytene (not Zygotene). The chiasmata (cross-shaped structures) are visible in Diplotene but are the sites where crossing over already occurred in Pachytene.

5. Mitosis vs Meiosis — Key Comparisons

Feature	Mitosis	Meiosis
Where occurs	Somatic cells	Germline cells (gonads)
Divisions	One	Two
Daughter cells	2, diploid (2n)	4, haploid (n)
Chromosome pairing	No synapsis	Synapsis in Meiosis I
Crossing over	No	Yes (Pachytene)
Genetic outcome	Identical	Varied (recombination)
Purpose	Growth, repair, asexual repro.	Sexual reproduction, gametes

NEET Bio Cell Cycle Notes

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Chapter note placement for Cell Cycle and Cell Division.

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5 focused sessions: cell cycle phases, mitosis events, meiosis overview, Prophase I substages, and mitosis vs meiosis comparison — 15 NEET-style MCQs each.

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NEET-style 60-question mock on Cell Cycle and Cell Division with timer, palette, answer review, and subtopic accuracy breakdown.

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