Bivalent (genetics)

Formation

The formation of a bivalent occurs during the first division of meiosis (in the zygotene stage of meiotic prophase 1). In most organisms, each replicated chromosome (composed of two identical sister chromatids) elicits formation of DNA double-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 (plural: chiasmata). 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.

This molecular machinery is involved in gene expression regulation in cells. During early organismal development, the coordinated activation of diverse transcriptional programs shapes organs and tissues. Bivalent promoters and poised enhancers are regulatory regions decorated with histone marks associated with both positive and negative transcriptional outcomes. Finally, this review notes the potential link between bivalency and cancer, which may inform research in disease etiology and treatment.