Chapter 13 - Chromatin structure and its effects on transcription. In this chapter, 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 Fifth 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. Chromatin Structure Eukaryotic genes do not exist naturally as naked DNA, or even as DNA molecules bound only to transcription factors They are complexed with an equal mass of other proteins to form chromatin Chromatin is variable and the variations play an enormous role in chromatin structure and in the control of gene expression 13- Histones Eukaryotic cells contain 5 kinds of histones H1 H2A H2B H3 H4 Histone proteins are not homogenous due to: Gene reiteration Posttranslational modification 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 . | Molecular Biology Fifth 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. Chromatin Structure Eukaryotic genes do not exist naturally as naked DNA, or even as DNA molecules bound only to transcription factors They are complexed with an equal mass of other proteins to form chromatin Chromatin is variable and the variations play an enormous role in chromatin structure and in the control of gene expression 13- Histones Eukaryotic cells contain 5 kinds of histones H1 H2A H2B H3 H4 Histone proteins are not homogenous due to: Gene reiteration Posttranslational modification 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, which have a core of histones, around which DNA winds 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
