During which phase of prophase I does crossing over occur, and why is this phase significant?

• A. Diplotene; it is significant because the nuclear envelope breaks down.
• B. Zygotene; it is significant because homologous chromosomes pair up.
• C. Leptotene; it is significant because chromatin condenses.
• D. Pachytene; it is significant because crossing over increases genetic diversity.

Answer: (D)