The pairing of homologues at the beginning of meiosis I ensures that each gamete receives one member of each pair. Homologues contact each other along much of their length and are held together by a special protein structure called the synaptonemal complex. This association of the homologues may persist from hours to days. The association of the two chromosomes is called a bivalent, and because there are four chromatids involved it is also called a tetrad. The points of attachment are called chiasmata (singular, chiasma).
The pairing of homologues brings together the near-identical sequences found on each chromosome, and this sets the stage for crossing over. The exact mechanism by which crossing over occurs is not known. Crossing over is controlled by a very large protein complex called a recombination nodule. Some of the proteins involved also play roles in DNA replication and repair, which is not surprising, considering that all three processes require breaking and reforming the DNA double helix.
One plausible model supported by available evidence suggests that crossing over begins when one chromatid is cut through, making a break in the double-stranded DNA (recall that each DNA strand is a double helix of nucleotides). A nuclease enzyme then removes nucleotides from each side of the DNA strand, but in opposite directions, leaving each side with a single-stranded tail, perhaps 600 to 800 nucleotides long.
One tail is then thought to insert itself along the length of one of the nonsister chromatids, aligning with its complementary sequence (i.e., if the tail sequence is ATCCGG, it aligns with TAGGCC on the nonsister strand). If a match is made, the tail pairs with this strand of the nonsister chromatid. This displaces the original paired strand on the nonsister chromatid, which is then freed to pair with the other single-stranded tail. The gaps are filled by a DNA polymerase enzyme. Finally, the two chromatids must be separated from each other, which requires cutting all the strands and rejoining the cut ends.
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