SME
Cell & Molecular Biology
Gene regulation
Use this for “Is the lac operon on or off with lactose and glucose both there?” or “Why does tryptophan turn the trp operon off?”
Explore
Both switches say no: the repressor holds the operator and CAP is off the DNA.
The gene is off: no enzymes are made for a sugar that is not there.
Assumptions
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An inducible operon (lac) is off until its inducer pulls the repressor off the operator.
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The lac operon also needs the activator CAP, which binds only with cAMP, and cAMP is high only when glucose is low.
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A repressible operon (trp) is on until its corepressor lets the repressor bind the operator.
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