# Cell Cycle and Cell Division

> Cell cycle phases G1, S, G2 and M, mitosis and meiosis events, crossing over and cytokinesis — NCERT notes for NEET-UG Biology.

- Canonical URL: https://prepelephant.com/topics/neet-ug/biology/cell-cycle
- Exam / course: NEET-UG · Subject: Biology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Cell Cycle and Cell Division", PrepElephant, https://prepelephant.com/topics/neet-ug/biology/cell-cycle

## Direct answer

The cell cycle is the ordered sequence of events by which a cell duplicates its contents and divides. Interphase — the metabolically active G1, S and G2 phases — occupies more than 95 per cent of the cycle duration, while the M phase includes mitosis (nuclear division) and cytokinesis (cytoplasmic division). DNA replication occurs in the S phase; mitosis keeps the chromosome number constant, whereas meiosis halves it for gamete formation.

## What you must remember

- **Interphase:** G1 (interval between mitosis and DNA replication), S (DNA synthesis — DNA content doubles but chromosome number stays the same), G2 (preparation for division, protein synthesis); a typical human cell cycle lasts about 24 hours, a yeast cell about 90 minutes.
- **Mitosis stages:** prophase (chromosome condensation), metaphase (chromosomes align at the metaphase plate, spindle fibres attach to the kinetochore), anaphase (centromeres split and sister chromatids move to poles), telophase (nuclear envelope re-forms).
- **Cytokinesis:** plant cells build a cell plate from the centre outward; animal cells develop a cleavage furrow that deepens centripetally.
- **Meiosis I:** zygotene (synapsis of homologous chromosomes, synaptonemal complex), pachytene (crossing over at recombination nodules), diplotene (chiasmata become visible); homologous chromosomes separate — the reductional division.
- **Meiosis II:** equational division in which sister chromatids separate, as in mitosis.
- **Significance:** mitosis grows and repairs tissues and maintains chromosome number in somatic cells; meiosis produces gametes, introduces variation through crossing over, and conserves the species chromosome number across generations.

## Common confusion

The classic error is chromosome number versus DNA content during S phase: after replication, DNA content doubles but the chromosome number does not change, because sister chromatids attached at one centromere still count as a single chromosome until anaphase splits the centromere. Equally, students confuse anaphase I with anaphase II — homologous chromosomes separate in meiosis I, sister chromatids in meiosis II.

## Exam-focused takeaway

NEET-UG loves statement-based questions such as "in S phase, DNA content doubles while chromosome number remains unchanged", and match-the-following between meiotic sub-stages and their events (synapsis-zygotene, crossing over-pachytene, chiasmata-diplotene). Assertion-reason items test why meiosis is called reductional but meiosis II equational, and when the centromere splits. Numerical or logic questions on chromosome and DNA counts at each stage appear almost every year, so practise counting chromatids through both divisions.

## Frequently asked questions

### In which phase of the cell cycle does DNA replicate?

In the S (synthesis) phase of interphase, during which the DNA content of the cell exactly doubles while the chromosome number remains the same.

### When do centromeres split during cell division?

During anaphase of mitosis and anaphase II of meiosis; centromeres do not split in anaphase I, where homologous chromosomes instead separate.

### Where does crossing over occur, and in which stage?

Crossing over occurs during pachytene of prophase I, at sites marked by recombination nodules; the visible proof of exchange is the chiasma seen in diplotene.

### How does cytokinesis differ in plant and animal cells?

Plant cells form a cell plate that grows outward to the periphery, while animal cells constrict through a cleavage furrow that deepens from the outside inward.

### Why is meiosis significant for a species?

It halves the chromosome number for gamete formation so fertilisation restores the diploid number, and crossing over generates the variation on which natural selection acts.