Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: During crossing-over, segments of DNA are exchanged between non-sister chromatids of homologous chromosomes. Notice how this can result in an allele (A) on one chromosome being moved to the other chromosome. The four chromatids compose the tetrad, with a chiasma at the point of exchange. Answer: during crossing-over
When does exchange of dna segments happen between non-sister chromatids of homologous chromosomes?