Final answer:
During prophase I of meiosis, homologous chromosomes are tightly bound in a structure known as a tetrad. The synaptonemal complex facilitates the exchange of genetic material, crossing over, creating chiasmata. By the end of prophase I, the synaptonemal complex breaks down and chromosomes, still connected at chiasmata, are visible as tetrads.
Step-by-step explanation:
During prophase I of meiosis, the chromosomal structure where all four chromatids of two homologues are closely associated is known as a tetrad. Initially, homologous chromosomes come together to form a synapse, tightly bound and aligned by a synaptonemal complex and cohesin proteins at the centromere. The synaptonemal complex supports the precise alignment of genes and enables crossing over, where chromosomal segments are exchanged between non-sister homologous chromatids, seen as chiasmata. This process significantly contributes to genetic diversity.
As prophase I advances, the synaptonemal complex begins to break down and chromosomes further condense. Homologous chromosomes start to separate but remain connected at chiasmata, which are the visible manifestations of crossing over. At the culmination of prophase I, the homologous chromosomes are visible as tetrads, held together by chiasmata, setting the stage for the subsequent stages of meiosis where they will be separated into different cells.