Crossing over occurs in; Meiosis, Prophase I.
The events that take place in phase, Prophase I are:
Each set of sister chromatids pair up with its homologous pair of chromosomes, forming a set of four chromatids.
• At several places along their length, non-sister chromatids (chromatids that belong to homologous chromosomes but are not identical sister chromatids) undergo a process called crossing over.
• The chromatids of homologous chromosomes cross over one another to hold the set of four chromatids together. Then the crossed sections of the chromatids are exchanged, forming new combinations of alleles in the cell.
• As prophase I continues, the cell prepares for a division of the nucleus like what happens in mitosis.
• Spindle fibers form between the centrioles, which are at opposite ends of the cell.
• The nuclear membrane and the nucleolus disappear.
Prophase I is the longest stage of meiosis, often taking up to 90% of the time required for the entire process to occur.