You have now seen both ways a eukaryotic cell divides its nucleus: Mitosis and Meiosis. Putting them side by side is the quickest way to keep them straight.
Key idea
Mitosis: one division, two identical diploid cells, for growth and repair. Meiosis: two divisions, four different haploid cells, with crossing over and independent assortment.
| Mitosis | Meiosis | |
|---|---|---|
| Divisions | 1 | 2 |
| Cells made | 2 | 4 |
| Chromosome number | Same as parent (2n → 2n) | Half the parent (2n → n) |
| Daughter cells | Identical to each other and the parent | Different from each other and the parent |
| Homologs pair up? | No | Yes, in prophase I (and may cross over) |
| What separates | Sister chromatids | Homologs (meiosis I), then sister chromatids (meiosis II) |
| Job | Growth, repair, replacing cells | Making gametes |
Why it works
One question sorts almost everything: what separates? If sister chromatids separate in the only division, the copies are shared out evenly and the daughters are identical (mitosis). If homologs separate first, each daughter gets one of each pair, half the number, and a different mix (meiosis I), and then the chromatids separate (meiosis II).
Worked example: Predict the products
A cell with $2n = 12$ divides. Give the number of cells and the chromosomes in each (a) after mitosis, (b) after meiosis.
- (a) Mitosis: 2 cells, each with the parent's number, 12.
- (b) Meiosis: 4 cells, each with half, $12 \div 2 = 6$.
Answer: (a) 2 cells of 12; (b) 4 cells of 6
Common mistake
Mixing the outcomes, such as "four identical cells" or "two haploid cells". Neither division makes those. Count the divisions (one or two) and ask what separates (homologs separate only in meiosis I). One division of sister chromatids gives two identical diploid cells; two divisions starting with homologs give four different haploid cells. Check with humans: mitosis turns one cell of 46 into two cells of 46, and meiosis turns one cell of 46 into four cells of 23.