In which stage of prophase I does crossing over occur?

Crossing over occurs during the pachytene stage of prophase I. This is a crucial part of meiosis where homologous chromosomes pair up closely in a process known as synapsis. In this stage, each chromosome consists of two sister chromatids, and the homologous chromosomes align together, allowing for the physical exchange of genetic material at specific points called chiasmata. This genetic recombination is vital for increasing genetic diversity in the offspring.

The process begins in the earlier stages of prophase I, such as leptotene, where chromosomes begin to condense, and zygotene, where homologous chromosomes start to pair up. However, the actual crossing over, which results in the exchange of genetic segments, takes place distinctly during the pachytene stage. Diakinesis is the final stage of prophase I where the chromosomes condense further and prepare for separation, but crossing over has already been completed by that point.