Final answer:
Crossing over during prophase I of meiosis is the exchange of genetic material between non-sister chromatids of homologous chromosomes, facilitated by the synaptonemal complex and resulting in genetic diversity.
Step-by-step explanation:
Crossing over is a crucial process during prophase I of meiosis where genetic material is exchanged between non-sister chromatids of homologous chromosomes. This genetic recombination contributes to genetic diversity in sexually reproducing organisms. It begins with the pairing of homologous chromosomes, each consisting of two sister chromatids, to form a tetrad. The synaptonemal complex, a protein structure, holds the homologs together along their length.
The exchange of genetic material results in recombination nodules, which indicate where the chiasmata, or points of crossover, will occur. DNA cleavage and reconnection at these nodules lead to an exchange of genetic segments. Once crossovers have occurred, the synaptonemal complex breaks down and chromosomes begin to condense, but remain connected at chiasmata. By the end of prophase I, tetrads are visible and held together only by these chiasmata, ensuring the proper segregation of chromosomes during the first meiotic division.