Meiosis I splits the pairs, meiosis II the sisters
- The idea
- Predict
- See it
- Why
- Check
The idea
“Meiosis I splits the sister chromatids, then meiosis II separates the homologous pairs.”
Mitosis splits sister chromatids, so it's easy to picture meiosis doing that first. Both orders end in four single-chromatid cells, so the swap is easy to miss.
The short version
The mistake: Meiosis I splits the sister chromatids, then meiosis II separates the homologous pairs.
What's actually true: Meiosis I separates the homologous pairs, and each chromosome keeps its sisters joined. Meiosis II separates the sisters.
- In meiosis I, homologs line up as pairs. The spindle pulls the partners to opposite poles, so each cell gets one of each pair.
- So meiosis I is the reductional division, from diploid, 2n, to haploid, n. Each chromosome still has two chromatids.
- Meiosis II works like mitosis. The sisters separate, giving four haploid cells with one chromatid per chromosome.
On the exam: Meiosis I separates homologs and halves the count. Meiosis II separates sisters.