18_Lecture_Post

18_Lecture_Post - Regulation of Gene Expression Overview:...

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Regulation of Gene Expression 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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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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Precursor Feedback inhibition Enzyme 1 Enzyme 2 Enzyme 3 Tryptophan (a) (b) Regulation of enzyme activity Regulation of enzyme production Regulation of gene expression trpE gene trpD gene trpC gene trpB gene trpA gene Figure 18.2
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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 The repressor prevents gene transcription by binding to the operator and blocking RNA polymerase
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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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Promoter DNA Regulatory gene mRNA trpR 5 3 Protein Inactive repressor RNA polymerase Promoter trp operon Genes of operon Operator mRNA 5 Start codon Stop codon trpE trpD trpC trpB trpA E D C B A Polypeptide subunits that make up enzymes for tryptophan synthesis (a) Tryptophan absent, repressor inactive, operon on (b) Tryptophan present, repressor active, operon off DNA mRNA Protein Tryptophan (corepressor) Active repressor No RNA made Figure 18.3
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Repressible and Inducible Operons: Two Types of Negative Gene Regulation A repressible operon is one that is usually ON; binding of a repressor to the operator shuts OFF transcription The trp operon is a repressible operon An inducible operon is one that is usually OFF; a molecule called an inducer inactivates the repressor and turns ON transcription
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The lac operon is an inducible operon and contains
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18_Lecture_Post - Regulation of Gene Expression Overview:...

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