Which statement best describes crossing over?

Which statement best describes crossing over?

• A. Sister chromatids exchange genetic material during meiosis II
• B. Homologous chromosomes exchange genetic material during prophase I
• C. Chromosomes duplicate during anaphase
• D. Spindle fibers attach to centromeres during prophase II

Answer: (B)

Crossing over is the genetic reshuffling that happens when homologous chromosomes exchange segments of DNA during meiosis. This occurs in prophase I, as homologous chromosomes pair up in synapsis to form a tetrad, and the non-sister chromatids of these homologs cross over at points called chiasmata, then reattach so that the chromatids carry new combinations of alleles. This timing and these partners—homologous chromosomes, not sister chromatids—are what create recombinant chromatids and increase genetic variation in gametes. Other statements miss the key idea: exchanging material between sister chromatids would not generate new combinations and crossing over doesn’t occur in meiosis II. Duplication of chromosomes happens earlier, during the S phase before meiosis begins, not during anaphase. Spindle fibers attaching to centromeres during prophase II concerns movement and alignment, not the exchange of genetic material.

Crossing over is the genetic reshuffling that happens when homologous chromosomes exchange segments of DNA during meiosis. This occurs in prophase I, as homologous chromosomes pair up in synapsis to form a tetrad, and the non-sister chromatids of these homologs cross over at points called chiasmata, then reattach so that the chromatids carry new combinations of alleles. This timing and these partners—homologous chromosomes, not sister chromatids—are what create recombinant chromatids and increase genetic variation in gametes.

Other statements miss the key idea: exchanging material between sister chromatids would not generate new combinations and crossing over doesn’t occur in meiosis II. Duplication of chromosomes happens earlier, during the S phase before meiosis begins, not during anaphase. Spindle fibers attaching to centromeres during prophase II concerns movement and alignment, not the exchange of genetic material.