Final answer:
The synaptonemal complex and cohesin proteins at the centromere hold the homologous chromosomes together as a bivalent during metaphase I of meiosis.
Step-by-step explanation:
During metaphase I of meiosis, the homologous chromosomes are held together as a bivalent by the synaptonemal complex and cohesin proteins at the centromere. In early prophase I, homologous chromosomes come together and form a synapsis, which is the precise pairing of the chromosomes due to the synaptonemal complex forming a lattice around them. As prophase I progresses, this lattice begins to break down, but the chromosomes remain attached at chiasmata, which are the visible points of crossing over. At the end of prophase I, the pairs of homologous chromosomes, now called tetrads, are held together only at the chiasmata.