Meiosis takes two divisions
- The idea
- Predict
- See it
- Why
- Check
The idea
“Meiosis I halves the chromosome number, so its two cells are the finished gametes.”
Meiosis I is the division that halves the chromosome number. Once 46 drops to 23, it's easy to think the cell is done.
The short version
The mistake: Meiosis I halves the chromosome number, so its two cells are the finished gametes.
What's actually true: Meiosis has two divisions. Meiosis I separates homologs, and meiosis II splits sister chromatids, making four cells.
- In S phase, before meiosis, the cell copies its DNA. Each chromosome then has two sister chromatids joined at the centromere.
- Meiosis I separates homologous chromosomes, the matching pair with one from each parent. The cells are haploid, but each chromosome still has two chromatids.
- Meiosis II then splits the sister chromatids, much like mitosis.
On the exam: Meiosis I halves the chromosome number. Meiosis II splits sister chromatids. A gamete needs both.