Mitosis keeps the chromosome number the same. But gametes must carry half the number: when two gametes combine, the new cell has two sets again. Meiosis is the cell division that makes those haploid cells.
Key idea
Meiosis is two divisions that turn one diploid cell into four haploid cells. Meiosis I separates homologous pairs, halving the chromosome number. Meiosis II separates sister chromatids, like mitosis.
| Stage | What happens | Chromosomes per cell ($2n = 4$) |
|---|---|---|
| Interphase | DNA is copied | 4 (8 chromatids) |
| Prophase I | Homologs pair up and may cross over | 4 |
| Metaphase I | Homologous pairs line up in the middle | 4 |
| Anaphase I | Homologs separate; sister chromatids stay together | 4 (moving to two poles) |
| Telophase I and cytokinesis | Two haploid cells | 2 in each (still 2 chromatids each) |
| Meiosis II (prophase II to telophase II) | In each cell, chromosomes line up singly and sister chromatids separate | 2 in each of four cells, single chromatids |
Why it works
The difference from mitosis is in metaphase I. In mitosis, chromosomes line up one by one; in meiosis I, they line up in pairs, each homolog facing a different pole. When the pairs split, each new cell gets one chromosome of each pair: a full single set, $n$. Each of those chromosomes is still two chromatids, so meiosis II splits them, exactly like mitosis, without copying the DNA again in between.
Worked example: Count through meiosis
A cell with $2n = 8$ goes through meiosis. How many chromosomes are in each cell after meiosis I, and after meiosis II?
- Meiosis I separates the 4 homologous pairs: each of two cells gets 4 chromosomes (each still two chromatids).
- Meiosis II separates sister chromatids: each of four cells gets 4 chromosomes of one chromatid.
- The number halved once, from 8 to 4, in meiosis I.
Answer: 4 after meiosis I; 4 after meiosis II (in four cells)
Common mistake
Thinking meiosis II halves the number again, or that meiosis I separates sister chromatids. The number halves only once, in meiosis I, when homologs part. Meiosis II separates sister chromatids, like mitosis, so the number of chromosomes per cell stays at $n$. Check with humans: 46 in, 23 per cell after meiosis I, still 23 after meiosis II.