Mitosis & meiosis

Copy a cell, or halve its chromosome set.

The idea

Both divisions follow an interphase in which DNA is replicated (S phase), so each chromosome enters division as two identical sister chromatids joined at a centromere. Mitosis is a single division — prophase, metaphase, anaphase, telophase (PMAT) — that pulls sister chromatids apart and produces two genetically identical diploid (2n) cells for growth, repair, and asexual reproduction. Meiosis is two divisions in a row that produce four genetically different haploid (n) gametes: in prophase I homologous chromosomes pair up and swap segments (crossing over), in metaphase I each homologous pair lines up in a random orientation (independent assortment), anaphase I separates the homologs, and anaphase II finally separates the sister chromatids. In humans that takes 46 chromosomes down to 23, and fertilization restores 46.

Work through an example

A human cell with 2n = 46 replicates its DNA and enters division with 46 chromosomes made of 92 chromatids. Mitosis gives two cells with 46 chromosomes each. Meiosis I gives two cells with 23 chromosomes (each still two chromatids), and meiosis II gives four cells with 23 single-chromatid chromosomes. Independent assortment alone allows 2²³ = 8,388,608 different chromosome combinations in one gamete, before crossing over adds more.

What to watch for

Chromosome number does not double in S phase — DNA content doubles, but a chromosome with two chromatids still counts as one chromosome, so count centromeres, not arms. Likewise, it is anaphase I, where homologous pairs separate, that halves the chromosome number (the reductional division); anaphase II separates sister chromatids exactly as mitosis does, and the number stays at n.