Ace A Level Biology 2026 – Dive Deep and Conquer Science!

The correct choice highlights a critical process in meiosis known as crossing over, which occurs at chiasmata. During prophase I of meiosis, homologous chromosomes pair up and form structures called tetrads. It's at the regions called chiasmata where the chromosomes physically exchange segments of genetic material. This exchange leads to genetic recombination, increasing genetic diversity in the resulting gametes. Without this process, the variability of the offspring would be significantly reduced, which is vital for evolution and adaptation.

In this context, DNA replication does not occur at chiasmata; rather, it takes place during the S phase of the cell cycle before meiosis begins. The separation of chromosomes occurs later in anaphase I and anaphase II of meiosis. Lastly, chromatids are duplicated prior to meiosis, during the S phase as well, rather than at the chiasmata stage during the meiotic process. Thus, crossing over is the specific event that accurately describes what happens at chiasmata during meiosis.