Heterochromatin is a type of chromatin (the chromosomal material) that is darkly staining and tightly packaged or coiled throughout the cell cycle and that is, for the most part, genetically inactive. There are two types of heterochromatin: constitutive heterochromatin and facultative heterochromatin .
Constitutive heterochromatin is fixed and irreversible in form and function. Chromosomes 1, 9, 16 and the human Y chromosome contain regions of constitutive heterochromatin. Formation of constitutive heterchromatin plays an important role in the long term changes in gene expression that constitute cell differentiation. Constitutive heterochromatin also tends to occur around the centromere of a chromosome.
Facultative heterochromatin has the ability to return to the euchromatic state. The inactive X chromosome is made up of facultative heterochromatin. Some facultative heterochromatin is thought to be inactivated by being attached to the nuclear envelope. Other possible mechanisms for its formation include chemical changes such as methylation and histone acetylation that prevent the DNA from being accessed by enzymes involved in the uncoiling process.
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