Meiosis

Meiosis makes haploid gametes (with half the chromosomes) from diploid cells. It has one interphase and two divisions: meiosis I is reductional (the chromosome number halves), and meiosis II is equational, like mitosis. The result is four haploid cells.

Prophase I is long and has five sub-stages:

• Leptotene: chromosomes appear as thin threads.
• Zygotene: matching (homologous) chromosomes pair up, helped by a protein structure, the synaptonemal complex, forming bivalents (tetrads).
• Pachytene: crossing over: non-sister chromatids exchange pieces at recombination nodules.
• Diplotene: the homologues begin to separate but stay joined at the crossover points, the chiasmata.
• Diakinesis: the chiasmata slide to the ends (terminalisation) and the nuclear envelope breaks down.

Metaphase I: the bivalents line up. Anaphase I: the homologous chromosomes separate, while each one's sister chromatids stay joined. Telophase I follows, then a short pause, interkinesis, with no DNA copying.

Meiosis II: chromosomes line up again; in anaphase II the sister chromatids separate; and four haploid nuclei form.

Why meiosis matters: it keeps the chromosome number constant from one generation to the next. And crossing over plus the independent sorting of chromosomes make every gamete genetically different, a major source of variation among offspring.