Prophase I

The prophase of meiosis I is an extended phase that is subdivided into five stages (see Fig. 2.64):

• Leptotene. The chromosomes become visible as thin strands.

• Zygotene. Homologous chromosomes of maternal and paternal origin pair. This pairing involves the formation of a synaptonemal complex, a tripartite structure that brings the chromosomes into physical association so that crossing-over may occur.

• Pachytene. As the chromosomes condense, the individual chromatids become visible. Crossing-over occurs early in this phase.

• Diplotene. The chromosomes condense further, and chi-asmata or contacts between the chromatids appear. The chiasmata indicate crossing-over may have occurred.

• Diakinesis. The chromosomes reach their maximum thickness, the nucleolus disappears, and the nuclear envelope disintegrates.

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