The tightly packed, condensed form of DNA in chromosomes that is typically transcriptionally inactive and difficult to access.
From Greek hetero- (different) + chroma (color) + -tin. Coined by Emil Heitz in 1928 to describe different staining patterns in electron microscopy of chromosomes.
Heterochromatin was one of biology's first 'dark matter' discoveries—scientists could see it was there but couldn't access it or read it, yet it turned out to be essential for keeping chromosomes stable and silencing troublemakers.
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