The crossing over is an actual exchange between homologous chromosomes. It means that it must occur when the homologous chromosomes are together in the prophase of meiosis. But prophase of meiosis is a lengthy phase and involves a number of stages. At what stage of meiosis prophase crossing over occurs. There can be two stages at which crossing over might occur:
1. Two-Strand Stage: The exchange of chromosome segments between homologous chromosomes could occurs before the chromosomes have duplicated.
2. Four-Strand Stage: The exchange of chromosome segment could take place at the four strand stage after each homologous chromosomes has separated into two chromatids.