From Wikipedia,
the free encyclopedia.
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.
External links
