Pachytene, also referred to as pachynema, is one of the five sub stages of Prophase I in meiosis. It is the third sub stage following leptotene and zygotene and preceding diplotene and diakinesis, beginning after synapsis is complete. Pachytene is defined as the stage when a fully formed synaptonemal complex exists.
During pachytene the homologous chromosomes thicken and become recombinant. Crossing over of segments of genetic information occurs between non-sister chromatids of homologous chromosomes, forming exchange sites called chiasmata and resulting in recombination. The process of chromosomal crossing over is crucial for accurate segregation as chiasmata hold the maternal and paternal chromosomes together.