Crossing over occurs during prophase I of meiosis, specifically in the pachytene stage, when homologous chromosomes pair up and exchange genetic material.

Core Process

Homologous chromosomes align as tetrads, allowing non-sister chromatids to break and swap segments at chiasmata points, creating genetic recombination. This shuffling boosts diversity in gametes, essential for evolution and variation in offspring.

Precise Timing

Within prophase I's sub-stages—leptotene, zygotene, pachytene, diplotene, diakinesis—crossing over peaks in pachytene after synapsis. DNA replication precedes this, ensuring chromatids are ready for exchange without loss.

Biological Significance

  • Generates novel allele combinations, breaking linkage and enabling new traits.
  • In humans, females average 55-60 events per meiosis, males 40-50, varying with age.
  • Errors can lead to disorders like Down syndrome via unequal crossing over.

Variations Across Life

Crossing over appears universal in sexually reproducing organisms but differs: more frequent in human females, hotspot-driven by PRDM9 gene. Plants and fungi show similar mechanics, though timing post-DNA replication varies slightly.

TL;DR : Crossing over happens in pachytene of prophase I during meiosis for genetic diversity.

Information gathered from public forums or data available on the internet and portrayed here.