In which phase does crossing over occur during meiosis?

• A. Metaphase I.
• B. Prophase I.
• C. Anaphase II.
• D. Telophase II.

Answer: (B)

Crossing over happens when homologous chromosomes pair up during meiosis I, specifically in prophase I. During this stage, the paired chromosomes undergo synapsis and exchange segments between nonsister chromatids, producing recombinant chromatids and increasing genetic diversity in gametes. By the time metaphase I arrives, the exchange has already occurred and the chromosomes are just lining up for separation; in the later stages, chromatids are pulled apart and no new recombination happens. So the phase where crossing over occurs is prophase I.