Final answer:
During prophase I of meiosis I, homologous chromosomes pair up to form tetrads and undergo crossing-over, which is the exchange of genetic material that increases genetic diversity. This process is facilitated by the synaptonemal complex and helps ensure normal chromosome segregation.
Step-by-step explanation:
Crossing-Over During Prophase I
Crossing-over is a hallmark of prophase I during meiosis I, where genetic material is exchanged between non-sister chromatids of homologous chromosomes. During this phase, homologous chromosomes pair up, aligning gene-for-gene, to form a structure known as a tetrad. The intimate contact between chromosomes allows sections of DNA to be swapped between chromosomal pairs, breaking off from one and reattaching to the homologous partner. This recombination results in chromosomes with a new combination of genes, increasing genetic diversity among the resulting gametes. Early in prophase I, a synaptic event happens where homologous chromosomes come together at precise points forming a synapse, aided by a protein structure called a synaptonemal complex and cohesin proteins at the centromere. The crossing-over process is critical for genetic variation and is essential for normal chromosome segregation during subsequent meiotic phases. Eventually, as prophase I advances, the chromosomes further condense and remain attached at sites known as chiasmata, manifesting as tetrads.