Final answer:
During metaphase I of meiosis, homologous chromosomes are held together as a bivalent by chiasmata, which are the result of crossover events that occur earlier in the process.
Step-by-step explanation:
During metaphase I of meiosis, homologous chromosomes are held together as a bivalent by structures known as chiasmata. The chiasmata are the physical manifestations of crossover, or genetic recombination, that occur between non-sister chromatids during the earlier stages of meiosis. Early in prophase I, the chromosomes are tightly bound together in perfect alignment primarily due to two elements: a protein lattice called the synaptonemal complex and cohesin proteins at the centromeres. The formation of recombination nodules along the synaptonemal complex mark where the chiasmata will later appear. As meiosis progresses, the synaptonemal complex breaks down, and by the end of prophase I, cohesion between homologous chromosomes is resolved, except at the chiasmata, ensuring that each homologous pair, now referred to as tetrads, remains connected until anaphase I.