pachytene

Pachytene is the third stage of prophase I in meiosis. It follows leptotene and zygotene and precedes diplotene. During this stage, homologous chromosomes are fully synapsed along their lengths by the synaptonemal complex, and each chromosome consists of two sister chromatids. The synaptonemal complex stabilizes pairing and promotes genetic recombination between non-sister chromatids.

Recombination is active during pachytene; crossing-over events form between homologous chromosomes and are visualized as chiasmata as meiosis proceeds. Recombination nodules are observed along the synapsed bivalents and are involved in resolving crossovers that generate genetic exchange. Chromosome condensation continues, and the chromosomes appear as tightly packed tetrads.

The synaptonemal complex is largely present during early to mid-pachytene and begins to disassemble later in the stage, with complete disassembly occurring by diplotene. The end of pachytene marks the beginning of synaptic dissolution, though chiasmata keep homologs connected until metaphase I.

Timing and duration of pachytene vary among species and sexes but generally last a substantial portion of prophase I. The transition to diplotene is characterized by the continued separation of homologs except at chiasmata, followed by diakinesis as chromosomes prepare for metaphase I.

Significance: Pachytene is a key phase for ensuring proper homologous recombination and accurate chromosome segregation in meiosis, contributing to genetic diversity and genome integrity.