Constitutive heterochromatin
Constitutive heterochromatin domains are regions of DNA found throughout the chromosomes of eukaryotes. The majority of constitutive heterochromatin is found at the pericentromeric regions of chromosomes, but is also found at the telomeres and throughout the chromosomes. In humans there is significantly more constitutive heterochromatin found on chromosomes 1, 9, 16, 19 and Y. Constitutive heterochromatin is composed mainly of high copy number tandem repeats known as satellite repeats, minisatellite and microsatellite repeats, and transposon repeats. In humans these regions account for about 200Mb or 6.5% of the total human genome, but their repeat composition makes them difficult to sequence, so only small regions have been sequenced.
Visualization of constitutive heterochromatin is possible by using the C-banding technique. The regions that stain darker are regions of constitutive heterochromatin. The constitutive heterochromatin stains darker because of the highly condensed nature of the DNA.
Constitutive heterochromatin is not to be confused with facultative heterochromatin, which is less condensed, less stable, and much less polymorphic, and which does not stain when using the C-banding technique.