Control of Gene Expression in Eukaryotes

Control of Gene Expression in Eukaryotes - Chapter 18 Gene...

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Chapter 18: Gene Expression in Eukaryotes Differential gene expression is responsible for creating different cell types, arranging tem into tissues, and coordinating their activity to form the multicellular society we call an individual. I. Mechanisms of Gene Regulation in Eukaryotes a. Eukaryotes can control the gene expression at the levels of transcription, translation, and post-translation. Three additional levels of control occur in eukaryotes as genetic information flows from DNA to proteins. DNA is wrapped around proteins to create a DNA protein complex called chromatin. Eukaryotic genes have promoters but before transcription can begin the stretch of DNA containing the promoter must be released from tight interactions with proteins, so that RNA polymerase can make contact with the promoter. Chromatin remodeling must occur prior to transcription. b. The second level of regulation that is unique to eukaryotes is RNA processing, these are the steps required to produce a mature processed mRNA from a primary RNA transcript. c. mRNA life span is regulated in eukaryotes. mRNAs that are active in the cell for a long time tend to be translated more than mRNAs that have a short life potential.
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II. Chromatin Remodeling a. What is Chromatin’s Basic Structure? i. The most abundant DNA associated proteins belong to a group called histones. Chromatin consists of DNA complexed with histones and other proteins. ii. The intimate association between DNA and histones occurs in part because DNA is negatively charged and histones are positively charged. DNA has a negative charge because of its phosphate groups; histones are positively charged because they contain many lysine or arginine resides or both. iii. A eukaryotic chromosome is made up if chromatin that has several layers of organization: the DNA is wrapped around histones to form nucleosomes; nucleosomes are packed into 30-nm fibers, 30nm fibers are attached to scaffold proteins and the entire assembly can be folded into the highly condensed structure observed during cell division. b. Evidence that Chromatin Structure is Altered in Active Genes The central idea is that chromatin must be relaxed or decondensed for RNA polymerase to bind to the promoter. Chromatin remodeling would represent the first step in the control of eukaryotic gene expression. i. “Closed” DNA in Protected from DNase 1. DNase is an enzyme that cuts DNA at random locations. The enzyme cannot cut DNA efficiently if the molecule is tightly complexed with histones. The enzyme works effectively only if DNA is in the “open” configuration. ii. Histone Mutants 1. The second type of evidence in support of the chromatin-remodeling hypothesis comes from studies of mutant brewer’s yeast cells that do not produce the usual complement of histones. In these mutant cells, many yeast genes that are normally never transcribed are instead transcribed at high levels at all times. 2.
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Control of Gene Expression in Eukaryotes - Chapter 18 Gene...

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