Brief introduction to epigenetic regulation including the the two states of chromatin: euchromatin and heterochromatin.
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 phosphorylation 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 acetyl transferases 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 nucleosomes which impedes transcription factors from gaining access to regulatory sites on the DNA. Methylation of cytosine bases and regions called CpG Islands is a hallmark of transcriptionally repressed heterochromatin. 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.