Bivalent chromatin
Bivalent chromatin is chromatin that contains both activating and repressing epigenetic modifications in the same area. Activating chromatin modifications increase the accessibility of the chromatin to RNA polymerase, where repressing modifications decrease the accessibility to RNA polymerase. Usually, these do not both occur at the same location, as it seems they would have countering effects; however in bivalent chromatin, they are both present. The repressing modifications usually take precedence, causing the gene to become inactivated. Once the repressing modifications are removed however, the activating modifications attract transcription machinery, and the gene becomes activated.
Bivalent chromatin is hypothesized to serve a role in genetic imprinting and in development.
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