Definition of Heterochromatin, constitutive

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Heterochromatin, constitutive: Heterochromatin that is fixed and irreversible. Regions of constitutive heterochromatin are located at very specific spots in the genome (on chromosomes 1, 9, 16 and the Y chromosome, the tiny short arms of chromosomes 13-15 and 21 and 22, and near the centromeres of chromosomes) and consists of DNA that contains many tandem (not inverted) repeats of a short basic repeating unit (known as satellite DNA).