The default state for the lac operon is "off" while the default state for the trp operon is "on". Explain this difference in

regulation and the mechanism of control via the repressor proteins



What will be an ideal response?


ANSWER: The lac operon's function is to allow a cell to use lactose as an energy source. Thus, the
operon only needs to be expressed when lactose is present in the environment. Consequently,
the Lac repressor normally binds to the operator, preventing transcription. However, when
lactose is present, allolactose will bind to the repressor, thereby preventing the repressor from
binding to the operator. When the repressor is not bound to the operator, RNA polymerase is
able to transcribe the lac operon.
The trp operon's function is to allow the bacterial cell to synthesize tryptophan, which is an
amino acid used as a building block in protein synthesis. The cell must make tryptophan
unless it is available in the environment. Therefore, the trp operon is on unless there is

Biology & Microbiology

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