Overview Composition DNA - charge histone proteins + charge from Lys and Arg residues H2A, H2B, H3, H4 (core proteins) H1 (linking protein) post-translational modification of histone tails forms a code interpreted by effector proteins to regulate transcription downstream histone acetyltransferase (HAT) acetyl group added which blocks positive charge of histone protein and loosens interaction with DNA ↑ transcription histone deacetylase (HDAC) removes acetyl group which exposes positive charge and tightens interactions ↓ transcription electrostatic attraction of DNA with histone proteins Organization 10 nm chromatin DNA wraps around dimer of H2A:H2B:H3:H4 called nucleosome sensitive to nuclease activity 30 nm chromatin nucleosomes held together by H1 not sensitive to nuclease activity 30 nm fiber loops further condensation Euchromatin/heterochromatin euchromatin = accessible to transcription 10 nm through 30 nm fiber loops heterochromatin = not accessible to transcription any greater condensation than 30 nm fiber loops condensed to save room