The big idea: Meiosis is the special cell division that makes gametes (sex cells — sperm and egg).
It takes one diploid (2n) cell and divides it twice to make four haploid (n) cells.
Because the chromosome number is halved, meiosis is called a reduction division. The four gametes it makes are all genetically different from each other.
One diploid cell (2n) divides twice: meiosis I separates the homologous chromosomes, meiosis II separates the sister chromatids, giving four haploid (n) gametes. Crossing over and independent assortment make the four gametes genetically different.
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- Meiosis
- A division that makes gametes: one diploid cell divides twice to form four haploid cells that are genetically different.
- Diploid (2n)
- A cell that has two copies of each chromosome — one set from each parent (in humans, 46 chromosomes = 23 pairs).
- Haploid (n)
- A cell that has only one copy of each chromosome — half the diploid number (in humans, 23 chromosomes). Gametes are haploid.
- Gamete
- A sex cell (sperm or egg) — haploid, so that two gametes can join at fertilisation to restore the diploid number.
- Homologous chromosomes
- A matching pair of chromosomes — the same size and carrying the same genes, one inherited from each parent (though the versions of the genes may differ).
- Reduction division
- A division that halves the chromosome number, turning a diploid cell into haploid cells. Meiosis is a reduction division.
Why halving matters: If gametes had the full diploid number, fertilisation would double the chromosome number every generation.
By halving it to haploid, meiosis makes sure that when a sperm and egg join, the offspring gets back the correct diploid number — and keeps it stable from one generation to the next.
Meiosis is two divisions in a row, not one.
In meiosis I, the homologous chromosomes (the matching pairs) are separated — this is where the chromosome number halves, so meiosis I is the reduction division.
In meiosis II, the sister chromatids are separated, much like mitosis, finishing the job and giving four haploid gametes.
| Feature | Meiosis I (first division) | Meiosis II (second division) |
|---|---|---|
| What is separated | Homologous chromosomes are pulled apart | Sister chromatids are pulled apart |
| Chromosome number | Halves: diploid (2n) → haploid (n) | Stays haploid (n) |
| This is the… | Reduction division (the number halves here) | Division that finishes splitting the chromatids |
| Cells made | Two haploid cells | Four haploid cells (the gametes) |
| Variation added | Crossing over (prophase I) + independent assortment (metaphase I) | (no new variation source examined here) |
Where meiosis happens: In animals, meiosis happens in the gonads — the testes (making sperm) and the ovaries (making eggs).
In a flowering plant, meiosis happens in the anthers (making pollen, the male gametes) and in the ovules inside the ovary (making the female gametes / egg cells).
Source 1 — crossing over (prophase I): Early in meiosis I (prophase I), the homologous chromosomes pair up and lie alongside each other.
Where they touch, they swap matching sections — this is crossing over.
Swapping sections mixes the alleles on each chromosome, so a chromosome ends up with a new combination it did not have before.
Source 2 — independent assortment (metaphase I): At metaphase I, each pair of homologous chromosomes lines up at the middle of the cell, and which one of each pair goes to which pole is decided at random.
This is independent assortment: every pair is sorted independently of the others, so the gametes get a fresh mix of maternal and paternal chromosomes.
Why the four gametes differ: crossing over (prophase I) swaps sections between homologous chromosomes, and independent assortment (metaphase I) shuffles which chromosome of each pair goes to each pole.
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Crossing over
- Happens in prophase I
- Homologous chromosomes pair up and swap sections
- Mixes the alleles along a chromosome
- Creates new combinations on each chromosome
Independent assortment
- Happens in metaphase I
- Each homologous pair lines up and sorts at random
- Shuffles whole maternal vs paternal chromosomes
- Every gamete gets a different mix of chromosomes
A memory hook: Crossing over crosses sections within a pair (prophase I).
Independent assortment sorts whole chromosomes between the poles (metaphase I).
Together they make every one of the four gametes genetically unique.
Meiosis vs mitosis: Don't confuse the two divisions. Mitosis makes two genetically identical body cells (no change in chromosome number). Meiosis makes four genetically different gametes and halves the chromosome number.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two (meiosis I then meiosis II) |
| Daughter cells made | Two | Four |
| Chromosome number | Stays the same (2n → 2n) | Halves (2n → n) — reduction division |
| Genetically… | Identical to the parent cell | Different from the parent and from each other |
| Makes | Body (somatic) cells for growth and repair | Gametes (sex cells) for reproduction |
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How this is tested: The flagship question here is a long Explain [7] on Paper 2: explain the stages and processes of meiosis that generate genetic variation — the markers want crossing over (prophase I), independent assortment (metaphase I), and the idea that two divisions give four genetically different gametes.
On Paper 1 a 1-mark question often asks what happens during the first division (meiosis I) — the answer is that homologous chromosomes are separated.
A short List [2] can ask for two structures in a flowering plant where meiosis occurs — the anthers and the ovules.
IB-style question — explain how meiosis generates variation
Explain how the stages and processes of meiosis generate genetic variation in the gametes produced. [5]
How to score all five marks
- Set the scene. Meiosis takes one diploid cell and divides it twice to make four haploid gametes.
- Crossing over. In prophase I, homologous chromosomes pair up and swap matching sections, mixing the alleles so each chromosome carries a new combination.
- Independent assortment. At metaphase I, each homologous pair lines up and is sorted to the poles at random, so gametes get different mixes of maternal and paternal chromosomes.
- Random combination. Because both processes are random, each of the four gametes ends up with a different combination of alleles and chromosomes — and fertilisation later combines two random gametes, adding even more variation. (Award 1 mark per distinct point: two divisions/four gametes; crossing over; prophase I; independent assortment; metaphase I / random; up to 5.)
Final answer
Meiosis divides one diploid cell twice into four haploid gametes; crossing over (prophase I) swaps sections between homologous chromosomes, and independent assortment (metaphase I) sorts each pair to the poles at random — so each gamete carries a different, unique combination of genes.
✓ Why this scores full marks: It names both variation sources, and ties each to its stage (crossing over → prophase I; independent assortment → metaphase I).
A common mistake is to name the processes but not say where they happen, or to describe only one of the two — an 'explain [5]' needs separate, linked points.
| Source of variation | When it happens | What it does |
|---|---|---|
| Crossing over | Prophase I | Homologous chromosomes pair up and swap matching sections, mixing the alleles on each chromosome |
| Independent assortment | Metaphase I | Each pair of homologous chromosomes lines up and is sorted to the poles at random, so gametes get a new mix of maternal and paternal chromosomes |