Chapter 18 Regulation of Gene Expression

Chapter 18 Regulation of Gene Expression - Chapter18

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Chapter 18 Regulation of Gene Expression
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Overview: Conducting the Genetic Orchestra Prokaryotes and eukaryotes alter gene expression in response to their changing environment In multicellular eukaryotes, gene expression regulates development and is responsible for differences in cell types RNA molecules play many roles in regulating gene expression in eukaryotes
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Concept 18.1: Bacteria often respond to environmental change by  regulating transcription Natural selection has favored bacteria that produce only the products needed by that cell A cell can regulate the production of enzymes by feedback inhibition or by gene regulation Gene expression in bacteria is controlled by the operon model
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Operons: The Basic Concept A cluster of functionally related genes can be under coordinated control by a single on-off “switch” The regulatory “switch” is a segment of DNA called an operator usually positioned within the promoter An operon is the entire stretch of DNA that includes the operator, the promoter, and the genes that they control The operon can be switched off by a protein repressor
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The repressor prevents gene transcription by binding to the operator and blocking RNA polymerase The repressor is the product of a separate regulatory gene The repressor can be in an active or inactive form, depending on the presence of other molecules A corepressor is a molecule that cooperates with a repressor protein to switch an operon off For example, E. coli can synthesize the amino acid tryptophan
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By default the trp operon is on and the genes for tryptophan synthesis are transcribed When tryptophan is present, it binds to the trp repressor protein, which turns the operon off The repressor is active only in the presence of its corepressor tryptophan; thus the trp operon is turned off (repressed) if tryptophan levels are high
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Concept 18.2: Eukaryotic gene expression can be  regulated at any stage All organisms must regulate which genes are expressed at any given time In multicellular organisms gene expression is essential for cell specialization
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Differential Gene Expression Almost all the cells in an organism are genetically identical Differences between cell types result from differential gene expression , the expression of different genes by cells with the same genome Errors in gene expression can lead to diseases including cancer Gene expression is regulated at many stages
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Regulation of Chromatin Structure Genes within highly packed heterochromatin are usually not expressed Chemical modifications to histones and DNA of chromatin influence both chromatin structure and gene expression
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Histone Modifications In histone acetylation , acetyl groups are attached to positively charged lysines in histone tails This process loosens chromatin structure, thereby promoting the initiation of transcription
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DNA Methylation
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Chapter 18 Regulation of Gene Expression - Chapter18

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