Cell Cycle and Cell Division: How One Cell Becomes Two
An easy walkthrough of interphase, mitosis and meiosis - the phases, DNA changes and why each matters - for NEET Biology.
By the PadhoDost Team · 📖 7 min read · Updated 4 August 2026
Part of NEET (Medical) prep🧠 A cell is like a photocopy shop
Before a busy photocopy shop can split into two branches, it must first stock up on paper, copy every important document exactly, and double-check the copies. Only then does it neatly divide everything into two identical new shops. A cell does the same: it grows and copies its DNA during interphase, then physically splits during the M phase.
The cell cycle has two main parts. Interphase is the long 'preparation' stage (about 95% of the cycle) with three sub-phases: G1 (the cell grows and is metabolically active but does not copy DNA), S (DNA replication - the DNA amount doubles, though the chromosome number stays the same), and G2 (the cell makes proteins and prepares to divide). The M phase (mitosis) is the short actual division. A typical human cell completes one cycle in about 24 hours, with mitosis taking only about an hour.
| Phase | What happens | DNA / chromosome status |
|---|---|---|
| G1 | Cell grows, makes RNA and proteins, is metabolically active | 2C DNA, 2n chromosomes (no copying yet) |
| S | DNA replication; each chromosome gains a sister chromatid | DNA doubles to 4C; chromosome number still 2n |
| G2 | More proteins made; cell prepares for mitosis | 4C DNA, 2n chromosomes |
| M (mitosis) | Nucleus and then cytoplasm divide into two | Each daughter cell returns to 2C, 2n |
Mitosis: the equational division
The four stages of mitosis (P-M-A-T)
- 1Prophase: chromosomes condense and become visible as two sister chromatids; the spindle starts forming and centrosomes move to opposite poles; the nuclear envelope and nucleolus begin to disappear.
- 2Metaphase: the nuclear envelope is gone; chromosomes line up at the equator (the metaphase plate) with spindle fibres attached to their kinetochores.
- 3Anaphase: centromeres split; sister chromatids separate and are pulled to opposite poles as daughter chromosomes.
- 4Telophase: chromosomes reach the poles and decondense; the nuclear envelope and nucleolus reappear, forming two nuclei. Cytokinesis (a cleavage furrow in animals, a cell plate in plants) then divides the cytoplasm.
Meiosis: the reductional division
Meiosis makes gametes (eggs, and sperm or pollen). It is called reductional because it halves the chromosome number - one diploid cell (2n) produces four haploid cells (n). This needs two rounds of division, Meiosis I and Meiosis II, but DNA is copied only once, before Meiosis I. The star event happens in Prophase I: homologous chromosomes pair up (synapsis) and swap segments in a process called crossing over during the pachytene stage, which shuffles genes and creates variation. In Anaphase I the homologous chromosomes separate; in Meiosis II the sister chromatids separate, just like in mitosis.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two (Meiosis I and II) |
| Daughter cells | 2 | 4 |
| Chromosome number | Stays the same (2n to 2n) | Halved (2n to n) |
| Genetic result | Identical to the parent | Genetically varied |
| Where it occurs | Body (somatic) cells - growth and repair | Germ cells - gamete formation |
| Crossing over | Absent | Present (in Prophase I) |
📝 Tracking a human cell (2n = 46)
Start (G1): 46 chromosomes, DNA = 2C.
After S phase: still 46 chromosomes, but each now has 2 sister chromatids, DNA = 4C.
After mitosis: two cells, each with 46 chromosomes and DNA = 2C - identical to the parent.
After Meiosis I: chromosome number halves to 23 (each still with 2 chromatids), DNA = 2C.
After Meiosis II: four cells, each with 23 chromosomes and DNA = 1C - these become the gametes.
At fertilisation, two gametes (23 + 23) fuse to restore 46, keeping the species' number constant.
Quick recap
- ✓Cell cycle = Interphase (G1, S, G2) + M phase; interphase is by far the longest part.
- ✓DNA is replicated only in the S phase; the chromosome number does not change during S.
- ✓Mitosis (PMAT) keeps the chromosome number the same and is used for growth and repair.
- ✓Meiosis halves the chromosome number and produces four genetically varied haploid gametes.
- ✓Crossing over happens in Prophase I (pachytene); homologues separate in Anaphase I; sister chromatids separate in Anaphase II.
- ✓Cytokinesis differs: a cleavage furrow in animal cells, a cell plate in plant cells.
⚡ Quick check
During which stage of mitosis do the centromeres split so that sister chromatids move to opposite poles?
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