Cell cycle, mitosis and meiosis !
- Process
The cell cycle is the sequence by which a cell grows, copies its DNA and divides; its brakes fail in cancer.
Updated 2026-11-01
What it is
The cell cycle has an interphase (G1, S and G2) and an M phase of division. Checkpoints make sure the DNA is intact and fully copied before the cell moves on. Mitosis makes two identical cells; meiosis, in the gonads, makes four different haploid gametes. Oncogenes and tumour suppressor genes control the process.
Step by step
- G1: the cell grows and does its job; this is the phase whose length varies most. Cells that stop dividing exit to G0.
- G1/S checkpoint: cyclin D with CDK4/6 phosphorylates Rb, which releases E2F and lets the cell enter S; p53 halts it if DNA is damaged.
- S: DNA is replicated and each chromosome ends up with two sister chromatids.
- G2: the cell checks the copy and gets ready to divide.
- Prophase and prometaphase: chromosomes condense, the nucleolus disappears, the nuclear envelope breaks down and the spindle forms.
- Metaphase: chromosomes line up on the equatorial plate; the spindle checkpoint requires every one to be attached to microtubules.
- Anaphase: sister chromatids move apart to the poles.
- Telophase and cytokinesis: two nuclei form and a ring of actin and myosin divides the cytoplasm.
Mitosis versus meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Where | Somatic cells | Germ cells (testis, ovary) |
| Divisions | One | Two (meiosis I reductional, meiosis II equational) |
| Result | Two identical diploid cells | Four different haploid cells |
| Recombination | No | Yes, crossing over in prophase I |
| Classic error | Aneuploidy in tumours | Nondisjunction: trisomy 21, usually in maternal meiosis I |
Clinical notes
- High-risk HPV: the E6 protein degrades p53 and E7 inactivates Rb, removing the cycle's brakes and leading to cervical cancer.
- Many anticancer drugs act in one phase: antimetabolites in S; vinca alkaloids and taxanes in M.
- Mature neurons and cardiac myocytes sit in G0: they barely regenerate by division.
In the lab
- Under the microscope a mitotic figure looks like very dark, condensed chromatin with no nuclear envelope; in metaphase, like a band or plate.
- The mitotic index and Ki-67 immunohistochemistry estimate how many cells in a tumour are dividing.
- Atypical mitoses (tripolar, for example) suggest malignancy.
Test yourself
Think of the answer, then tap the question to see it.
What does the Rb protein do in the cell cycle?
It holds the E2F factor and brakes entry into S phase; once phosphorylated by cyclin D-CDK4/6 it releases E2F and the cycle moves on.
In which phase is DNA copied?
In the S phase of interphase.
Which HPV proteins inactivate p53 and Rb?
E6 promotes the breakdown of p53 and E7 binds and inactivates Rb.
In which meiotic division do homologous chromosomes separate?
In meiosis I, which halves the chromosome number.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- Cytoskeleton: microfilaments, intermediate filaments, microtubules
- Cell injury, apoptosis and necrosis
- Cell adaptations: hypertrophy, hyperplasia, atrophy, metaplasia
- Testis: seminiferous tubules, Sertoli and Leydig cells, spermatogenesis
Related conditions
Microorganisms
In Pharmacology
Educational content on normal microscopic structure and its clinical relevance; no substitute for studying slides or for a health professional's advice.
Sources: 3.5 Cell Growth and Division (OpenStax (Rice University), 2022); What Is Cancer? (National Cancer Institute, 2021).
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Representative model, for educational purposes only.