Crossing over occurs during prophase I of meiosis. During this stage, homologous chromosomes pair up closely to form structures called tetrads. It is at this time that nonsister chromatids of homologous chromosomes exchange segments of genetic material. This exchange results in new combinations of alleles, contributing to genetic diversity in the resulting gametes. Crossing over is completed before the cell proceeds to metaphase I, where tetrads line up at the cell's equator for subsequent separation. To summarize:
- Crossing over happens in prophase I of meiosis.
- Homologous chromosomes form tetrads and exchange genetic material between nonsister chromatids.
- This genetic recombination increases diversity among gametes.
- By metaphase I, crossing over is completed, and the chromosomes then segregate during anaphase I.
This process is crucial for genetic variation and is a defining feature of meiosis compared to mitosis.
