Topic 20, lac operon

Topic 20, lac operon - Topic 20: the lac operon An example...

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Topic 20: the lac operon An example of “inducible” gene expression
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Similarities: Differences: DNA replication and RNA transcription compared: Characteristic: Template DNA: --one or both strands used? Newly made nucleic acids: --roughly how long are they? --single or double-stranded? --how do incorporated nucleotides compare --how do sequences compare to sense strand? How the polymerase works: --continuous vs. discontinuous? --how does polymerase know where to bind? --How big is the “repl/transc. bubble”? How does polymerase know to stop? Is a primase needed?
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Be able to define the following terms: open reading frame (ORF), monocistronic, polycistronic, operon, operator, repressor, inducer Be able to define and differentiate the terms: inducible, repressible, constitutive gene expression Be able to explain and/or diagram how the lac operon works to allow a bacterial cell to begin to use lactose as an energy source Know the roles of each molecule listed on slide 27 Also be able to explain why and how the presence of glucose also regulates the lac operon Be able to describe the relative activity of transcription of the lac operon based upon any combination of lactose or glucose (slide 30) Learning objectives
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Terms to understand: Open Reading Frame (ORF): a DNA sequence that contains a start codon and stop codon (usually implies that a protein is produced) Monocistronic: a mRNA that encodes a single ORF/polypeptide chain (protein) Polycistronic: a single mRNA that encodes multiple ORFs/polypeptide chains/proteins Operon: 2 or more contiguous ORFs under the control of a single operator Operator: a component of a promoter that gives additional control over whether or not gene is actively transcribed
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Types of methods to control gene expression: Inducible: only turn these genes on when they are necessary (normally off) Repressible: turn genes off when they are no longer necessary (normally on) Constitutive: gene is always on, no matter what
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Figure 14.01a: Negative regulation. Negative regulation: gene is normally active, unless a repressor molecule binds to inactivate gene expression (example of repressible gene expression , e.g. trp operon)
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Figure 14.01b: Positive regulation Positive regulation: gene is normally inactive, unless an activator molecule binds to activate gene expression (example of inducible gene expression , e.g. lac operon)
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The lactose (lac) operon of E. coli Why discuss this? A classical model of inducible gene expression
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Topic 20, lac operon - Topic 20: the lac operon An example...

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