During which phase do chromosomes pair up and crossing over occurs?

The phase during which chromosomes pair up and crossing over occurs is Prophase I. During this stage of meiosis, homologous chromosomes (each composed of two sister chromatids) come together to form pairs in a process known as synapsis. This pairing is crucial for genetic diversity, as crossing over allows for segments of DNA to be exchanged between homologous chromosomes. This exchange results in new allele combinations, which contributes to the genetic variation seen in gametes. As Prophase I progresses, the structure of these paired chromosomes becomes more complex as they undergo synapsis and homologous recombination, preparing for the subsequent stages of meiosis.