During which stage of meiosis does crossing over take place?

Crossing over occurs during Prophase I of meiosis. This is a critical process where homologous chromosomes pair up closely and exchange segments of their genetic material. This exchange enhances genetic diversity in gametes, which is essential for evolution and adaptation in populations.

During Prophase I, homologous chromosomes align along their lengths in a process called synapsis, forming a structure known as a tetrad. It is within these tetrads that physical exchanges of chromosome segments can occur through a mechanism called homologous recombination. This is facilitated by the formation of chiasmata, the points where the chromatids of homologous chromosomes are joined together.

There are no genetic exchanges happening in Metaphase I, and by Anaphase II and Telophase II, the chromosomes have already been separated and the processes are focused on the division and formation of new cells, lacking the conditions necessary for crossing over to take place. Thus, Prophase I is the specific stage where crossing over is not only possible but is actively occurring, significantly contributing to genetic variation in the resultant gametes.