What is crossing-over and when does it occur?

Crossing-over is a crucial genetic process that occurs during prophase I of meiosis. It involves the exchange of genetic material between homologous chromosomes, which are pairs of chromosomes consisting of one chromosome from each parent. This exchange occurs at points called chiasmata, where the chromatids can break and rejoin.

The significance of crossing-over lies in its contribution to genetic diversity; it results in new combinations of alleles on each chromosome, which can lead to variations in traits among offspring. This variance is essential for evolution and adaptation within populations. By reshuffling the genetic material, crossing-over enhances the genetic variation passed on during sexual reproduction.

The other options pertain to different processes: the formation of sister chromatids is associated with mitosis, the separation of homologous chromosomes occurs during anaphase of meiosis, and DNA replication takes place during the S phase of the cell cycle. Thus, these options do not accurately describe crossing-over or the stage of meiosis in which it occurs.