Epigenetic regulation encompasses
a number of different modifications to chromatin.
These include methylation of the DNA on cytosine bases,
a modification that can further be oxidized,
as well as modification of the histone tails
that emanate from the core of the nucleosome.
The tails of core histones labeled here
can be altered with distinct chemical modifications
including methylation of histone H3, acetylation of histone H4,
and phoshorylation of histone H2B.
Euchromatin is often characterized
by a more open and accessible state of the DNA, one in which
transcription factors have access
to their cognate binding sites and can,
therefore, recruit enzymes, like histone acetyltransferases that
acetylate histone tails and activate genes
by recruiting components of the basal transcriptional
machinery, including RNA polymerase.
Heterochromatin, in contrast, is thought
to be characterized by a more repressive tight bundling
of nuclear zones, which impedes transcription
factors from gaining access to regulatory sites on the DNA.
Methylation of cytosine bases in regions
called CpG islands is a hallmark of transcriptionally repressed
These methylated cytosines in turn recruit proteins
like MeCP2, Methyl-CpG-binding Protein 2, and HP1,
Heterochromatin Protein 1.
These proteins are thought to maintain a repressive state
of chromatin by inducing histone deacetylation by HDACs,
as well as histone tail methylation
by histone methyltransferase enzymes.