During meiosis, bivalents are a pair of associated homologous chromosomes formed after replication. Each replicated chromosome comprises two chromatids.
During meiosis, bivalents are made up of a pair of homologous dyads. A dyad is a pair of sister chromatids with an undivided centromere. A dyad is formed after DNA replication during the pre-meiotic interphase. The actual bivalent forms during prophase I: homologous chromosomes commence synapsis during zygonema, and complete synapsis at pachynema. The bivalent is maintained by a ribbon-like proteinaceous structure called the synaptonemal complex, which is situated between the synapsed homologous chromosomes. The homologous chromosomes de-synapse during diplonema and diakinesis; however, the bivalent is maintained into metaphase I by one or more chiasmata, which are the sites at which crossing over occurred. The homologous chromosomes fully separate at anaphase I.
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