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A bivalent, sometimes referred to as a tetrad, is a pair of associated homologous chromosomes held together by a complex after chromosome replication. During meiosis (the Prophase I: Pachynema stage of Meiosis I), the process of synapsis occurs in which bivalents are formed. Each replicated chromosome is composed of two chromatids.
Bivalents are formed as two homologous chromosomes undergo recombination. Chromosome movement to accommodate recombination places homologous chromosomes close enough to each other for a protein complex (synaptonemal complex), consisting of a protein (ZIP1/ZYP1 depending on species), to hold them together. This protein's sequence is not well conserved, though its structure is very similar in all species (globular domains on the N and C termini with a well defined core). A tetrad is made up of four chromatids or two pairs of sister chromatids.
In the atypical model species D. melanogaster and C. elegans recombination occurs secondarily to synapsis.
Recombination produces chiasmata which hold chromosomes together after the synaptonemal complex dissolves. (Prophase 1 substage Diplotene)
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