Which statement best describes crossing over during meiosis?

Which statement best describes crossing over during meiosis?

• A. Replication of DNA prior to meiosis
• B. An exchange of genetic material between homologous chromosomes during prophase I
• C. Separation of sister chromatids during mitosis
• D. Random fertilization occurs

Answer: (B)

Crossing over is the exchange of genetic material between homologous chromosomes during prophase I of meiosis. At this stage, paired homologs align and their nonsister chromatids cross over and swap DNA segments, producing recombinant chromatids with new allele combinations. This event, visible at the sites called chiasmata, increases genetic variation in gametes by mixing genetic material from each parent. Replication of DNA happens earlier, in the S phase before meiosis, and isn’t the swapping event itself. Separation of sister chromatids occurs during mitosis or at the later stage of meiosis II, not during crossing over. Random fertilization describes how gametes from different individuals combine to form a zygote, another source of variation but not the chromosomal exchange process.

Crossing over is the exchange of genetic material between homologous chromosomes during prophase I of meiosis. At this stage, paired homologs align and their nonsister chromatids cross over and swap DNA segments, producing recombinant chromatids with new allele combinations. This event, visible at the sites called chiasmata, increases genetic variation in gametes by mixing genetic material from each parent.

Replication of DNA happens earlier, in the S phase before meiosis, and isn’t the swapping event itself. Separation of sister chromatids occurs during mitosis or at the later stage of meiosis II, not during crossing over. Random fertilization describes how gametes from different individuals combine to form a zygote, another source of variation but not the chromosomal exchange process.