What is crossing over and when does it occur?

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

Answer: (A)

Crossing over is the exchange of segments between non-sister chromatids of homologous chromosomes during meiosis I. It happens as the homologous chromosomes pair up in prophase I (during synapsis, especially in the pachytene stage), at sites called chiasmata, where pieces of DNA are swapped. This recombination creates new allele combinations, increasing genetic variation in the gametes.

So, crossing over occurs in prophase I, not in the later steps like metaphase II, anaphase I, or telophase II, where chromosomes are just aligning or separating without exchanging genetic material.