OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

During the crossing over stage of meiosis, chromosome pairs, or homologous chromosomes, exchange genetic material. This process occurs during prophase I of meiosis, where segments of DNA are swapped between non-sister chromatids of homologous chromosomes. This exchange leads to genetic recombination, resulting in new combinations of alleles on each chromosome. This genetic diversity is crucial for the evolution of species and enhances the adaptability of organisms. The other options do not accurately describe what happens during crossing over. Replication of chromosomes occurs earlier in the cell cycle during the S phase, pulling apart refers to anaphase where sister chromatids are separated, and dissolution of chromosomes does not occur in this context; chromosomes remain structured throughout meiosis. Overall, the primary purpose of crossing over is to create variation in the gametes produced, which is essential for sexual reproduction.