5.1 Meiosis

01Purpose

Meiosis makes haploid gametes (n) from a diploid cell (2n). Two divisions after one round of DNA replication produce four genetically different cells. In humans: 46 → 23.

02Meiosis I vs. II

Meiosis I separates homologous chromosomes (the reduction division). In prophase I homologs pair as tetrads (synapsis) and swap segments (crossing over). In metaphase I they line up in pairs; anaphase I pulls homologs apart.

Meiosis II looks like mitosis: sister chromatids separate. Result: 4 haploid cells.

03Notebook box

Mitosis: 1 division, 2 identical diploid cells, no synapsis. Meiosis: 2 divisions, 4 unique haploid cells, synapsis + crossing over in prophase I.

Worked example: A cell with 2n = 8 undergoes meiosis. How many chromosomes per cell after meiosis I? → 4 (each still with two chromatids).

Key takeaways

✦Meiosis I separates homologs; II separates sisters.
✦Four haploid, genetically unique cells.
✦Crossing over happens in prophase I.

Watch outChromosome number halves after meiosis I, not meiosis II.

Quick check

Did it stick?

1.Homologous chromosomes separate during…

2.A human gamete has how many chromosomes?

3.Crossing over occurs during…