During which phase does crossing over primarily occur?

The correct choice is Prophase I because this is the stage of meiosis where homologous chromosomes pair up and exchange genetic material through a process called crossing over. Crossing over occurs when certain non-sister chromatids of homologous chromosomes physically exchange segments of their DNA. This process is crucial for genetic diversity as it creates new combinations of alleles that can be passed on to the next generation.

During Prophase I, the chromosomes condense, the nuclear envelope breaks down, and the homologous chromosomes come together in a process known as synapsis. This physical pairing allows the precise alignment necessary for the exchange of genetic fragments to occur. The significance of crossing over cannot be overstated, as it contributes significantly to the variation seen in offspring, which is a fundamental aspect of evolution and natural selection.

This mechanism is not present in the other phases of meiosis listed. Metaphase II focuses on the alignment of chromosomes at the equatorial plate, Anaphase I is concerned with the separation of homologous chromosomes, and Telophase I involves the formation of two new nuclei, none of which involve the crossing over of genetic material between chromosomes.