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Combination of Switches: the Lac Operon

Is the lac operon like a light bulb connected to two switches? The lac operon is controlled by two switches one triggered by the presence of glucose and the other by the presence of lactose. When a light bulb is controlled by two switches there are four possible orientations: switch A and switch B both up, switch A and switch B both down, switch A up and switch B down, or switch A down and switch B up. The first two of these orientations will result in the light being on and the last two orientations will result in the light being off. In contrast only one orientation of switches will turn on the lac operon - both switches must be on.

View the animation below, then complete the quiz to test your knowledge of the concept.






1The lac operon is expressed when
A)glucose is high and lactose is present.
B)glucose is high and lactose is absent.
C)glucose is low and lactose is present.
D)glucose is low and lactose is absent.
E)glucose is low, regardless of the presence or absence of lactose.



2When in a complex with ________, the CAP protein binds to the CAP site and ________ the expression of the lac operon.
A)glucose ; switches on
B)glucose ; switches off
C)lactose ; switches on
D)cAMP ; switches on
E)cAMP ; switches off



3In the absence of lactose, the lac repressor is
A)active and can bind to the operator.
B)active and cannot bind to the operator.
C)inactive and cannot bind to the promoter.
D)inactive and can bind to the operator.
E)inactive and cannot bind to the operator.



4The inducer of the lac operon is an isomer of lactose.
A)True
B)False



5When RNA polymerase is bound to the lac promoter sequence, transcription of the lac operon is on.
A)True
B)False







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