Meiosis I

Meiosis makes gametes (eggs and sperm) with half the usual chromosome number (haploid). When two gametes join at fertilisation, the full (diploid) number is restored. Meiosis has two divisions (meiosis I and II) but the DNA is copied only once. Matching chromosomes (homologues) pair up and swap pieces, and the result is four haploid cells.

Prophase I is long and has five stages. A memory trick: "Lahrati Zulfon Par Dil Deewana" gives Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis.

• Leptotene: the chromosomes become visible as long threads.
• Zygotene: homologous chromosomes pair up side by side (synapsis), held by a protein ladder, the synaptonemal complex. Each pair is a bivalent (also called a tetrad, since it has four chromatids).
• Pachytene: the four chromatids are clearly seen. Small recombination nodules appear, where crossing over happens: non-sister chromatids of the two homologues exchange pieces. The enzyme involved is recombinase. This mixes the genes.
• Diplotene: the synaptonemal complex dissolves and the homologues begin to separate, but stay joined at the cross-over points, which look like an X: the chiasmata. In the egg cells of some animals this stage can last for months or years.
• Diakinesis: the chiasmata slide to the ends (terminalisation); the spindle forms; the nucleolus and nuclear envelope disappear.

Metaphase I: the bivalents line up on the equatorial plate. Anaphase I: the two homologues of each pair separate and move to opposite poles, but each one's sister chromatids stay together. Telophase I: nuclear membranes and nucleoli reappear, and the cell divides: the two cells are a dyad.

The short gap before meiosis II is interkinesis. There is no DNA copying in it.