A bivalent is one pair of chromosomes in a tetrad. A tetrad is the association of a pair of homologous chromosomes physically held together by at least one DNA crossover. This physical attachment allows for alignment and segregation of the homologous chromosomes in the first meiotic division.
Formation
The formation of a bivalent occurs during the first division of meiosis. In most organisms, each replicated chromosome elicits formation of DNAdouble-strand breaks during the leptotene phase. These breaks are repaired by homologous recombination, that uses the homologous chromosome as a template for repair. The search for the homologous target, helped by numerous proteins collectively referred as the synaptonemal complex, cause the two homologs to pair, between the leptotene and the pachytene phases of meiosis I. Resolution of the DNA recombination intermediate into a crossover exchanges DNA segments between the two homologous chromosomes at a site called a chiasma. This physical strand exchange and the cohesion between the sister chromatids along each chromosome ensure robust pairing of the homologs in diplotene phase. The structure, visible by microscopy, is called a bivalent.
Structure
A bivalent is the association of two replicated homologous chromosomes having exchanged DNA strand in at least one site called chiasmata. Each bivalent contains a minimum of one chiasma and rarely more than three. This limited number is due to crossover interference, a poorly understood phenomenon that limits the number of resolution of repair events into crossover in the vicinity of another pre-existing crossover outcome, thereby limiting the total number of crossovers per homologs pair.
Function
At the meiotic metaphase I, the cytoskeleton puts the bivalents under tension by pulling each homolog in opposite direction. The anchorage of the cytoskeleton to the chromosomes takes place at the centromerethanks to a protein complex called kinetochore. This tension results in the alignment of the bivalent at the center of the cell, the chiasmata and the distal cohesion of the sister chromatids being the anchor point sustaining the force exerted on the whole structure. Impressively, human femaleprimary oocytes remains in this tension state for decades, highlighting the robustness of the chiasma and the cohesion that hold the bivalents together.
