tailieunhanh - Lecture Biology: Chapter 18 - Niel Campbell, Jane Reece

Chapter 18 - Regulation of gene expression. When you finish this chapter, you should: Explain the concept of an operon and the function of the operator, repressor, and corepressor; explain the adaptive advantage of grouping bacterial genes into an operon; explain how repressible and inducible operons differ and how those differences reflect differences in the pathways they control;. | Chapter 18 Regulation of Gene Expression Overview: Conducting Gene Expression 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. Regulation of gene expression trpE gene trpD gene trpC gene trpB gene trpA gene (b) Regulation of enzyme production (a) Regulation of enzyme activity Enzyme 1 Enzyme 2 Enzyme 3 Tryptophan Precursor Feedback inhibition Figure Regulation of a metabolic pathway 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 - blocks transcription. The repressor prevents gene transcription as it binds to the operator and blocks 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 By default the trp operon is on and the genes for tryptophan synthesis are transcribed. When tryptophan is abundantly present, it binds to the trp repressor protein, which turns the operon off. The trp operon is a repressible operon: turned off, repressed, if tryptophan levels are high. Polypeptide subunits that make up enzymes for tryptophan synthesis (b) Tryptophan present, repressor active, operon off. Tryptophan (corepressor) (a) Tryptophan absent, repressor inactive, operon on. No RNA made Active repressor mRNA Protein DNA DNA | Chapter 18 Regulation of Gene Expression Overview: Conducting Gene Expression 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. Regulation of gene expression trpE gene trpD gene trpC gene trpB gene trpA gene (b) Regulation of enzyme production (a) Regulation of enzyme activity Enzyme 1 Enzyme 2 Enzyme 3 Tryptophan Precursor Feedback inhibition Figure Regulation of a metabolic pathway 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

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