When does crossing over occur?

• A. In prophase II
• B. In metaphase I
• C. In prophase I
• D. In anaphase II

Answer: (C)

Crossing over is the process where homologous chromosomes exchange segments between non-sister chromatids, creating new combinations of genes. This happens when the homologs pair up during meiosis I, as they undergo synapsis in prophase I. The exchange of genetic material is most active during the pachytene stage of prophase I, and the resulting chiasmata can be seen as evidence of this recombination as meiosis progresses.

Because this pairing and exchange require homologous chromosomes to align and trade segments, crossing over does not occur in later stages like prophase II, metaphase I, or anaphase II. In prophase II the cell has already completed the first division and the chromosomes are no longer paired with their homologs. In metaphase I the homologs are aligned for separation but the exchange has already happened, and in anaphase II the sister chromatids are being pulled apart, not exchanged.

So, the stage where crossing over occurs is prophase I, during the period of homolog pairing and genetic exchange between non-sister chromatids. This recombination is a major source of genetic diversity.