Pachytene

Pachytene is the third stage of prophase I in meiosis. It follows leptotene and zygotene, and precedes diplotene. During pachytene, several key events occur that are crucial for genetic diversity.

The defining characteristic of pachytene is the completion of synapsis, where homologous chromosomes are fully paired along their entire length. This pairing is facilitated by the synaptonemal complex, a protein structure that holds the homologous chromosomes together in close proximity.

A significant event during pachytene is genetic recombination, specifically crossing over. This process involves the exchange of genetic material between non-sister chromatids of homologous chromosomes. The physical locations where crossing over occurs are called chiasmata (although chiasmata are visually observed later in diplotene). Crossing over results in new combinations of alleles on the chromosomes, increasing genetic variation in the offspring.

Pachytene is generally the longest stage of prophase I. The proper alignment and pairing of chromosomes during this stage are essential for ensuring accurate segregation of chromosomes during subsequent meiotic divisions. Errors during synapsis or crossing over can lead to aneuploidy (an abnormal number of chromosomes) in the resulting gametes.

The end of pachytene is marked by the disassembly of the synaptonemal complex, which allows the homologous chromosomes to begin to separate in the next stage, diplotene.