In the lac operon, the presence of lactose in the bacterial environment leads to the expression of genes necessary for lactose metabolism. This regulation occurs through a specific molecular interaction. Which of the following best describes the direct effect of lactose on the lac operon?
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The answer
Correct answer: A. Lactose binds to the repressor protein, inducing a conformational change that prevents its binding to the operator region.
Tested concept: The lac and trp operons
Explanation
The lac operon is an inducible operon. In the absence of lactose, the lac repressor protein binds to the operator region, blocking RNA polymerase from transcribing the structural genes. When lactose is present, it is converted to allolactose, which acts as an inducer. Allolactose binds to the lac repressor protein, causing a conformational change that reduces the repressor's affinity for the operator. This allows the repressor to detach from the operator, enabling RNA polymerase to proceed with transcription of the lac operon genes. Distractor 1 is incorrect because lactose does not directly interact with RNA polymerase to enhance its affinity. Distractor 2 is incorrect because lactose (allolactose) binds to the repressor, not directly to the operator. Distractor 3 is incorrect because lactose acts as an inducer, not a corepressor; a corepressor (like tryptophan in the trp operon) would increase the repressor's affinity for the operator, thereby turning gene expression OFF.
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