During meiosis, which stage is crossing over most associated with?

• A. Prophase I
• B. Prophase II
• C. Metaphase I
• D. Anaphase II

Answer: (A)

Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes, creating new allele combinations. This recombination happens when homologous chromosomes pair up, which occurs in Prophase I of meiosis. During this stage, chromosomes condense and align in synapsis to form tetrads, with the synaptonemal complex holding them together. The actual crossing over occurs as DNA segments are exchanged at points called chiasmata, mainly during the pachytene phase. After Prophase I, homologs separate in Metaphase I and Anaphase I, and no new crossing over occurs in Prophase II or the later stages. Prophase II involves no pairing of homologs, so recombination doesn’t take place there. Therefore, Prophase I is the stage most associated with crossing over because it is the only time when homologous chromosomes pair and exchange genetic material.