During which stage of meiosis does crossing-over occur?

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

Answer: (A)

Crossing-over is the exchange of genetic material between nonsister chromatids of homologous chromosomes, which occurs during meiosis I as homologous chromosomes pair up in synapsis. This pairing forms tetrads, and the actual exchange of DNA segments happens in prophase I, especially during the pachytene stage when chromosomes are fully aligned and recombination machinery creates crossovers at chiasmata. This event generates recombinant chromatids and increases genetic variation in gametes. After crossing-over, the chromosomes continue through meiosis I to metaphase I and beyond. The later stages listed do not involve crossing-over because chromosome pairing and exchange have already occurred earlier in prophase I.