The chromosome number halves in meiosis I
- The idea
- Predict
- See it
- Why
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The idea
“The cell stays diploid until meiosis II, where the number halves.”
Meiosis II comes right before gametes form, so it seems like the step that makes them haploid. After meiosis I, each chromosome still has two chromatids, so the cell looks diploid.
The short version
The mistake: The cell stays diploid until meiosis II, where the number halves.
What's actually true: Meiosis I splits homologous pairs, so each new cell is already haploid. Meiosis II splits sister chromatids and keeps the number.
- DNA is copied before meiosis. A 2n = 4 cell still has 4 chromosomes, each now with two sister chromatids.
- Meiosis I separates homologous chromosomes, the matching members of a pair, one from each parent. Each new cell gets one of every pair, so it's haploid.
- Meiosis II separates sister chromatids, much like mitosis. Each chromatid becomes a chromosome, so the number stays the same.
- A haploid cell holds only one of each pair. Two chromatids per chromosome just means the DNA was copied.
On the exam: The number halves when homologs split, in meiosis I. Meiosis II only splits sister chromatids.