What happens during crossing over?

• A. Alleles are lost.
• B. Chromosomes duplicate.
• C. During meiosis, homologous chromosomes may exchange DNA.
• D. Sister chromatids separate.

Answer: (C)

Crossing over is the exchange of genetic material between homologous chromosomes during meiosis, specifically in prophase I when paired homologs come together (synapsis). Non-sister chromatids of these homologs physically break and rejoin, so segments are swapped at points called chiasmata. This recombination creates new combinations of alleles on each chromosome, increasing genetic diversity in the gametes. It doesn’t involve losing genetic material, nor does it involve duplication (that happens earlier in the cell cycle during S phase), and it doesn’t describe the separation of sister chromatids (that occurs later, during anaphase II). So the statement that best describes crossing over is that homologous chromosomes may exchange DNA during meiosis.