What does crossing over during prophase I accomplish?

• A. Homologous chromosomes exchange genetic segments, creating new allele combinations.
• B. Chromosomes replicate.
• C. Chromatids separate.
• D. Spindle forms.

Answer: (A)

Crossing over during prophase I creates new allele combinations by swapping genetic material between homologous chromosomes. When homologs pair up in synapsis, non-sister chromatids exchange segments at points called chiasmata, so each chromosome that goes into a gamete ends up with a mix of parental alleles. This recombination increases genetic diversity among offspring because the gene combinations in gametes aren’t exact copies of either parent.

Think about what’s happening in the cell cycle: the DNA was already replicated earlier in S phase, so the exchange isn’t about duplicating DNA but about reshuffling it. The actual separation of chromatids occurs later in meiosis, and spindle formation is just part of the machinery that moves chromosomes, not about creating new allele combinations.