Video Transcript

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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.

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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,

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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.

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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

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by histone methyltransferase enzymes.