Essential Key Points: Meiosis Divisions
Overall Picture: 1 diploid cell (2n, 4C after S phase) → Meiosis I → 2 haploid cells (n, 2C) → Meiosis II → 4 haploid gametes (n, 1C)
Meiosis I — Reductional Division
- Key event: Homologous chromosomes SEPARATE (not sister chromatids)
- Anaphase I: centromeres do NOT split; sister chromatids remain joined
- Result: 2 cells, each with n chromosomes and 2C DNA
- Why "reductional": chromosome number reduced from 2n to n
- Unique features vs. mitosis: synapsis, bivalents, crossing over, chiasmata, independent assortment
Meiosis II — Equational Division
- Like mitosis: sister chromatids separate in anaphase II
- Centromeres DO split in anaphase II
- No DNA replication between meiosis I and II (no interphase S phase)
- Result: 4 cells total, each with n chromosomes and 1C DNA
DNA Content Tracker:
- Before meiosis (G2): 4C, 2n
- After meiosis I: 2C, n (per cell; 2 cells)
- After meiosis II: 1C, n (per gamete; 4 cells total)
Key Comparative Table:
| Feature | Anaphase of Mitosis | Anaphase I | Anaphase II |
|---|---|---|---|
| What separates? | Sister chromatids | Homologous chromosomes | Sister chromatids |
| Centromere splits? | YES | NO | YES |
| Chromosome number change | 2n → 4n (transient) | 2n → n (at each pole) | n → n (at each pole) |
Sources of Genetic Variation in Meiosis:
- Crossing over (pachytene) — intrachromosomal recombination
- Independent assortment (metaphase I) — interchromosomal recombination
- Random fertilisation (not meiosis itself, but downstream)