crossing over prevents homologous chromosomes from separating during meiosis i. target 1 of 4 crossing over occurs at the ends of chromosomes, rather than near the centromeres. target 2 of 4 for crossing over to occur, homologous chromosomes must align precisely early in prophase i so that nonsister chromatids can exchange corresponding segments of dna. target 3 of 4 as a result of crossing over, sister chromatids are no longer identical to each other.