A genetically inactive part of the genome, heterochromatin was so named because it was chromosomal material (chromatin) that stained differently, more darkly, all through the cell cycle, than most chromosomal material (which was named euchromatin). There are two types, namely constituitive heterochromatin and facultative heterochromatin.
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The part of the chromonema that remains tightly coiled and condensed during interphase and thus stains readily. SYN: heteropyknotic chromatin.
- constitutive h. repetitive h. that lies in secondary constrictions in the nucleolar organizers.
- facultative h. nonrepetitive h. that comprises translatable sequences of DNA.
- satellite-rich h. h. that codes for 18 S and 28 S components of ribosomal RNA and is located close to the centromeres of certain chromosomes.

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het·ero·chro·ma·tin -'krō-mət-ən n densely staining chromatin that appears as nodules in or along chromosomes and contains relatively few genes

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chromosome material (see chromatin) that stains most deeply when the cell is not dividing. It is thought not to represent major genes but may be involved in controlling these genes, and also in controlling mitosis and development. Compare euchromatin.

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het·ero·chro·ma·tin (het″ər-o-kroґmə-tin) [hetero- + chromatin] the form of chromatin that is darkly staining and tightly coiled in interphase; it is composed of repetitive DNA, is late to replicate, and is transcriptionally inactive. Cf. euchromatin.

Medical dictionary. 2011.

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