Prophase I

Prophase I is longer and more complex than the prophase of mitosis. By how the chromosomes behave it is split into five stages, in order: leptotene, zygotene, pachytene, diplotene and diakinesis.

Leptotene: the chromosomes gradually become visible under the light microscope, and they keep compacting all through this stage.

Zygotene: homologous chromosomes begin to pair up, a process called synapsis. Electron micrographs show that synapsis comes with a complex structure, the synaptonemal complex.

Two homologues joined by synapsis form a bivalent, also called a tetrad; bivalents become clearer in the next stage. Leptotene and zygotene are short compared with pachytene.

Pachytene: the four chromatids of each bivalent become distinct and clearly appear as tetrads. Recombination nodules appear; these are the sites where crossing over happens between non-sister chromatids of the homologues.

Crossing over means homologous chromosomes swap pieces of genetic material. It is enzyme-mediated, the enzyme being recombinase, and it recombines the genetic material on the two chromosomes.

Recombination is finished by the end of pachytene, and the homologues stay linked at the crossover sites.

Diplotene begins when the synaptonemal complex dissolves. The recombined homologues of each bivalent tend to move apart, except where they crossed over; these X-shaped links are the chiasmata. In some vertebrates, an oocyte can stay in diplotene for months or even years.

Diakinesis, the last stage, is marked by terminalisation of the chiasmata. Condensation of the chromosomes is now complete, and the meiotic spindle is assembled to get the homologues ready to separate.

By the end of diakinesis the nucleolus has disappeared and the nuclear envelope has broken down; diakinesis is the transition into metaphase I.