Prophase I:
- Prophase I of the first meiotic division is divided into five phases- leptotene, zygotene, pachytene, diplotene, and diakinesis.
- In the leptotene phase, the chromosomes become visible and their compaction continues.
- During zygotene, the homologous chromosomes pair together and a protein complex called ‘synaptonemal complex’ tightly joins the paired chromosomes.
- Zygotene is followed by the pachytene in which crossing over takes place between non-sister chromatids of the homologous chromosomes.
- The beginning of the diplotene is marked by the dissolution of the synaptonemal complex and the propensity of the recombined homologous chromosomes to start separating from each other except at the crossover sites. This results in a formation of an X-shaped structure called ‘chiasmata’.
- The final phase (diakinesis) is characterized by chiasmata terminalization. The chromosomes become fully condensed. At the end of diakinesis, the nucleolus disappears and the nuclear envelope breaks down.
Figure: Different stages of prophase I- (1) leptotene, (2) zygotene, (3) pachytene, (4) diplotene, (5) diakinesis