00:04.51
Epigenetic regulation encompasses
00:06.61
a number of different modifications to chromatin.
00:10.29
These include methylation of the DNA on cytosine bases,
00:14.71
a modification that can further be oxidized,
00:18.18
as well as modification of the histone tails
00:20.67
that emanate from the core of the nucleosome.
00:28.22
The tails of core histones labeled here
00:30.89
can be altered with distinct chemical modifications
00:34.58
including methylation of histone H3, acetylation of histone H4,
00:43.43
and phoshorylation of histone H2B.
00:48.61
Euchromatin is often characterized
00:50.65
by a more open and accessible state of the DNA, one in which
00:54.40
transcription factors have access
00:56.38
to their cognate binding sites and can,
00:58.69
therefore, recruit enzymes, like histone acetyltransferases that
01:03.05
acetylate histone tails and activate genes
01:06.55
by recruiting components of the basal transcriptional
01:09.43
machinery, including RNA polymerase.
01:14.43
Heterochromatin, in contrast, is thought
01:17.40
to be characterized by a more repressive tight bundling
01:20.94
of nuclear zones, which impedes transcription
01:24.09
factors from gaining access to regulatory sites on the DNA.
01:28.70
Methylation of cytosine bases in regions
01:31.16
called CpG islands is a hallmark of transcriptionally repressed
01:36.11
heterochromatin.
01:37.67
These methylated cytosines in turn recruit proteins
01:40.97
like MeCP2, Methyl-CpG-binding Protein 2, and HP1,
01:48.26
Heterochromatin Protein 1.
01:51.30
These proteins are thought to maintain a repressive state
01:54.09
of chromatin by inducing histone deacetylation by HDACs,
02:01.88
as well as histone tail methylation
02:04.22
by histone methyltransferase enzymes.
