In Chapter 13, we will look at the crucial relationship among activators, chromatin structure, and gene activity. This chapter presents the following content: Histones, nucleosomes, chromatin structure and gene activity. | Molecular Biology Fourth Edition Chapter 13 Chromatin Structure and Its Effects on Transcription Lecture PowerPoint to accompany Robert F. Weaver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Histones Eukaryotic cells contain 5 kinds of histones H1 H2A H2B H3 H4 Each histone type isn’t homogenous Gene reiteration Posttranslational modification Source: Panyim and Chalkley, Arch. Biochem. & Biophys. 130, 1969, f. 6A, . 13- Properties of Histones Abundant proteins whose mass in nuclei nearly equals that of DNA Pronounced positive charge at neutral pH Most are well-conserved from one species to another Not single copy genes, repeated many times Some copies are identical Others are quite different H4 has only had 2 variants ever reported 13- Nucleosomes Chromosomes are long, thin molecules that will tangle if not carefully folded Folding occurs in several ways First order of folding is the nucleosome X-ray diffraction . | Molecular Biology Fourth Edition Chapter 13 Chromatin Structure and Its Effects on Transcription Lecture PowerPoint to accompany Robert F. Weaver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Histones Eukaryotic cells contain 5 kinds of histones H1 H2A H2B H3 H4 Each histone type isn’t homogenous Gene reiteration Posttranslational modification Source: Panyim and Chalkley, Arch. Biochem. & Biophys. 130, 1969, f. 6A, . 13- Properties of Histones Abundant proteins whose mass in nuclei nearly equals that of DNA Pronounced positive charge at neutral pH Most are well-conserved from one species to another Not single copy genes, repeated many times Some copies are identical Others are quite different H4 has only had 2 variants ever reported 13- Nucleosomes Chromosomes are long, thin molecules that will tangle if not carefully folded Folding occurs in several ways First order of folding is the nucleosome X-ray diffraction has shown strong repeats of structure at 100Å intervals This spacing approximates the nucleosome spaced at 110Å intervals 13- Histones in the Nucleosome Chemical cross-linking in solution: H3 to H4 H2A to H2B H3 and H4 exist as a tetramer (H3-H4)2 Chromatin is composed of roughly equal masses of DNA and histones Corresponds to 1 histone octamer per 200 bp of DNA Octamer composed of: 2 each H2A, H2B, H3, H4 1 each H1 13- H1 and Chromatin Treatment of chromatin with trypsin or high salt buffer removes histone H1 This treatment leaves chromatin looking like “beads-on-a-string” The beads named nucleosomes Core histones form a ball with DNA wrapped around the outside DNA on outside minimizes amount of DNA bending H1 also lies on the outside of the nucleosome 13- Nucleosome Structure Central (H3-H4)2 core attached to H2A-H2B dimers Grooves on surface define a left-hand helical ramp – a path for DNA winding DNA winds almost twice around the histone core condensing DNA length by 6- .
