Crossing over occurs between which structures?

• A. Sister chromatids during prophase II
• B. Homologous chromosomes during prophase I
• C. Homologous chromosomes lining up during meiosis I
• D. Crossing over occurs between homologous chromosomes

Answer: (B)

Crossing over happens when non-sister chromatids of homologous chromosomes exchange genetic material, and this occurs during prophase I of meiosis when the homologs pair up in synapsis. This exchange creates new combinations of alleles and increases genetic diversity in gametes. The description that best fits is that crossing over occurs between homologous chromosomes during prophase I, because it specifies both the partner (non-sister chromatids of homologous chromosomes) and the exact stage. The other options are less accurate: crossing over between sister chromatids would not generate new allele combinations and isn’t the typical event; prophase II isn’t when homologous chromosomes pair and cross over; and a vague statement about crossing over between homologous chromosomes misses the key detail that the exchange occurs between non-sister chromatids rather than the entire chromosome pair.