DNA methylation, histone acetylation, and chromatin structure. DNA ispackaged as chromatin, the basic subunit of which is the nucleosome. Eachnucleosome consists of two turns of DNA wrapped around a core histone of histoneproteins, the tails of which protrude. Transcriptionally active chromatin ischaracterized by unmethylated DNA and acetylated (green triangles) histone tails.(a) The DNA is exposed and accessible to transcription factor binding.(b) Methylation of cytosine bases in the DNA (red dots) attractsmethylcytosine-binding proteins which in turn attract and tether chromatininactivation complexes containing histone deacetylases and other proteins. (c) These complexes deacetylate the histones and promote condensation of thechromatin into a compressed structure inaccessible to the transcription initiationcomplexes. DNMT, DNA methyltransferase; HAT, histone acetylase; HDAC, histonedeacetylase; MBD, methyl-CpG-binding domain; RNA-PII, RNA polymerase II; TF,transcription factor. Figure reprinted with permission from Michigan Creative.