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Crossing over during prophase I is a vital mechanism that enhances genetic diversity among organisms undergoing meiosis. During this phase, homologous chromosomes—pairs of chromosomes, one from each parent—align closely together in a process known as synapsis. When they align, sections of the chromatids can exchange genetic material at points called chiasmata. This recombination leads to new combinations of alleles on the chromosomes, which results in gametes (sperm and egg cells) that have genetic variations different from the parent organisms.

This genetic shuffling is essential for evolution and adaptability, as it increases the potential for variation within a population. Such variation is important for natural selection and contributes to the overall genetic diversity of a species.

The other choices do not contribute directly to genetic diversity during meiosis. DNA replication is necessary for producing the chromosomes for meiosis but does not create variation. The splitting of sister chromatids in meiosis II is part of the process that ensures gametes receive the correct number of chromosomes but does not increase genetic diversity. Finally, cytokinesis’s role in reducing chromosome numbers is crucial for forming haploid cells, but like DNA replication, it does not directly result in genetic diversity.