Mitosis and meiosis
The two types of cell division and what each is for.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 17.3.1Describe mitosis and meiosis and state their roles.Supplement
5 minute read
Mitosis and meiosis
Both are cell division, but they serve opposite purposes — one preserves the genome, the other shuffles it.
Mitosis
Produces two genetically identical daughter cells, each with the same chromosome number as the parent (diploid). Used for growth, repair of damaged tissue, replacement of worn-out cells, and asexual reproduction. Before division, the DNA is replicated exactly, so each daughter cell receives a complete identical copy.
Meiosis
Produces four genetically different cells, each with half the chromosome number (haploid). Used only to make gametes. Two features generate variation:
- Chromosomes are shuffled so each gamete gets a different combination.
- Which gamete fuses at fertilisation is random.
Why halving is essential
If gametes were diploid, fertilisation would double the chromosome number every generation. Meiosis halves it so that fusion of two haploid gametes restores the normal diploid number — 46 in humans, from two gametes of 23.
Comparison
| Mitosis | Meiosis | |
|---|---|---|
| Daughter cells | 2 | 4 |
| Genetically | identical | different |
| Chromosome number | same (diploid) | halved (haploid) |
| Purpose | growth, repair, asexual reproduction | gamete formation |
The link to earlier work: mitosis produces clones, which is why asexual reproduction gives no variation; meiosis is one of the two sources of variation in sexual reproduction.
Think of it like this
Mitosis is photocopying a document — two identical copies. Meiosis is dealing a shuffled deck into four hands, each holding half the cards and no two alike.
Worked examples
Method, step by step
Human body cells contain 46 chromosomes. State the number in a human sperm cell and explain why.
- 1Sperm are gametes, produced by meiosis.
- 2Meiosis halves the chromosome number, producing haploid cells.
- 3Half of 46 is 23.
- 4At fertilisation, 23 from the sperm and 23 from the egg restore the diploid number of 46.
23 — meiosis halves the chromosome number so that fertilisation restores the normal diploid number of 46.
Common misconceptions
- Thinking meiosis produces two cells. It produces **four**, each haploid.
- Believing mitosis creates variation. It produces genetically identical cells; only mutation introduces change.
- Saying gametes are diploid. They are **haploid** — that is the entire point of meiosis.
In the exam
- If a question mentions growth, repair or asexual reproduction, the answer is mitosis. If it mentions gametes or variation, it is meiosis.
- For "why must gametes be haploid", state that fertilisation would otherwise double the chromosome number each generation.