Question

The expression of the tryptophan operon is controlled by a repressor that is active when it...

The expression of the tryptophan operon is controlled by

a repressor that is active when it is alone.

a repressor that is inactive when it binds to lactose.

a repressor that is active when it binds to tryptophan.

an activator that turns the operon on by binding to DNA.
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Answer #1

The correct answer is a repressor that is active when it binds to tryptophan. The the tryptophan operon is controlled by repressor. The tryptophan binds to the repressor causing it to change its shape and repressor gains its active state. The repressor is is bound to DNA to achieve its active state. The tryptophan blocks the RNA polymerase from binding to the promoter and hence preventing transcription in operon.

Option 1 is incorrect cause the repressor is not alone when its active. It is active when it is is bound to a DNA.

Option 2 is incorrect because in tryptophan operon the the presence of tryptophan causes activation of the repressor. Presence of lactose has no effect on this operon.

Option 4 is incorrect because the operon is not turned on by any activator by binding to DNA. Instead the the operon is activated by a DNA binded repressor.

Thank you. Please do like the answer.

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