What are the requirements and consequences of crossing over?
1. For crossing over to occur, homologous chromosomes must align precisely early in prophase I so that nonsister chromatids can exchange corresponding segments of DNA.
2. Crossing over occurs at the ends of chromosomes, rather than near the centromeres, because segments of DNA near the centromeres cannot break and rejoin easily.
3. As a result of crossing over, sister chromatids are no longer identical to each other.
4. Crossing over prevents homologous chromosomes from separating during meiosis I.