When does crossing over occur during meiosis?

Crossing over occurs during prophase I of meiosis, which is a crucial step in the process of genetic recombination. During this phase, homologous chromosomes pair up closely in a process called synapsis. Once they are aligned, the chromatids from maternal and paternal chromosomes can exchange segments of genetic material at points known as chiasmata. This exchange results in new combinations of alleles, which increases genetic diversity among gametes.

The significance of crossing over lies in its ability to shuffle alleles between homologous chromosomes, which is vital for evolution and adaptation in populations. Additionally, crossing over ensures that gametes produced during meiosis are genetically distinct, which enhances genetic variation in sexually reproducing organisms. This variation is fundamental in the processes of natural selection and evolution.