Fine Structure and Evolution of DNA in Heterochromatin

Abstract

Heterochromatin was recognized in the eukaryote chromosome some 50 years ago (Heitz 1929). This original definition contrasted heterochromatin to euchromatin in terms of heteropycnosis, heterochromatin remaining in a deeply staining, condensed state during the entire mitotic cycle. Subsequent research has shown other differences in that this ubiquitous chromosome component generally replicates later than the euchromatic regions of the genome, and in general shows no evidence of transcriptional activity. The existence of obvious heterochromatic polymorphisms in many species, and the observations in Drosophila that gross deficiencies of heterochromatin are tolerated with little or no phenotypic effects, have led to the erroneous conclusion that it is a genetically inert region of the chromosome.