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LECTURE CONNECTIONS 17 | Control of Gene Expression in Eukaryotes © 2009 W. H. Freeman and Company 17.1 Eukaryotic Cells and Bacteria Have Many Features of Gene Regulation in Common, but They Differ in Several Important Ways • Each structural gene has its own promoter, and is transcribed separately. • DNA must unwind from the histone proteins before transcription. • Activators are more common in Eukaryotes. • Transcription and translation are separated in time and space. 17.2 Changes in Chromatin Structure Affect the Expression of Genes • DNase I hypersensitive sites: more open chromatin configuration site, upstream of the transcription start site. At least 3 different processes affect gene regulation by altering chromatin structure: 1) Histone modification • Addition of methyl groups to the histone protein tails (activation or repression of transcription) • Addition of acetyl groups to histone proteins (usually stimulates transcription) 17.2 Changes in Chromatin Structure Affect the Expression of Genes 2) Chromatin remodeling • Chromatin-remodeling complexes: bind directly to DNA sites and reposition nucleosomes 3) DNA Methylation • DNA methylation of cytosine bases adjacent to guanine nucleotides (CpG)–CpG islands 17.3 The Initiation of Transcription Is Regulated by Transcription Factors and Transcriptional Activator Proteins • Trancriptional activators: stimulate and stabilize the basal transcription apparatus at the core promoter. • The activators may interact directly with the basal transcription apparatus or indirectly through protein coactivators. • Repressors: bind to sequences in the regulatory promoter or to distant sequences called silencers, inhibiting transcription. 17.3 The Initiation of Transcription Is Regulated by Transcription Factors and Transcriptional Activator Proteins • Transcriptional activator protein GAL4 (regulates the transcription of several yeast genes in galactose metabolism) Concept Check 1 Most transcriptional activator proteins affect transcription by interacting with . a. b. c. d. introns the basal transcription apparatus DNA polymerase nucleosomes 17.3 The Initiation of Transcription Is Regulated by Transcription Factors and Transcriptional Activator Proteins • Enhancers and insulators • Enhancer: DNA sequence stimulating transcription from a distance away from promoter • Insulator: DNA sequence that blocks or insulates the effect of enhancers in a position-dependent manner. 17.3 The Initiation of Transcription Is Regulated by Transcription Factors and Transcriptional Activator Proteins • Coordinated gene regulation • Response elements: common regulatory elements upstream of the start sites of a collective group of genes in response to a common environmental stimulus • Heat-shock proteins 17.4 Some Genes Are Regulated by RNA Processing and Degradation • Gene regulation through RNA splicing 17.4 Some Genes Are Regulated by RNA Processing and Degradation • The degradation of RNA (by ribonucleases) • 5′-cap removal • Shortening of the poly(A) tail • Degradation of 5′ UTR, coding sequence, and 3′ UTR 17.5 RNA Interference Is an Important Mechanism of Gene Regulation • Cleavage of mRNA • Inhibition of translation • Transcriptional silencing- altering chromatin structure, RITS (RNA transcription silencing) • Degradation of mRNA. • Small interfering RNAs and microRNAs • RISC: RNA-induced silencing complex 17.5 RNA Interference Is an Important Mechanism of Gene Regulation • Mechanisms of Gene regulation by RNA interference • RNA cleavage: RISC containing an siRNA, pair with mRNA molecules and cleavage to the mRNA • Inhibition of translation • Transcriptional silencing: altering chromatin structure 17.6. Some Genes Are Regulated by Processes That Affect Translation or by Modification of Proteins • Ribosomes, aminoacyt tRNAs, initiation factors and elongation factors are all required for the translation of mRNA molecules. • The activation of T lymphocytes (T cells) is critical to the development of immune responses to viruses.