When glucose is present, and lactose is absent in the bacteria cell’s environment, which of the following is INCORRECT with respect to the lac operon?

Question

BlackTom AI · Accepted Answer

The scenario describes a bacterial cell with glucose present and lactose absent. In this situation, the lac operon is shut down through two main mechanisms: low cAMP levels prevent CAP from activating transcription, and the lac repressor is bound to the operator because lactose is not available to inactivate it.

Option 1: 'CAP does NOT bind to its binding site within the operon' — This is consistent with low cAMP when glucose is present, so CAP remains inactive and does not bind. Therefore this statement is true in this context.

Option 2: 'cAMP levels are low in the cell' — This is also true, since high glucose suppresses adenylate cyclase activity, lowering cAMP and preventing CAP activation.

Option 3: 'The lacZ structural gene is NOT expressed' — With the repressor bound (lactose absent), RNA polymerase cannot initiate transcription of the operon, so lacZ is not expressed. This statement is true.

Option 4: 'The expression of the lacI gene is repressed.' — This statement is not accurate in this condition. The lacI gene, which encodes the repressor, is typically expressed constitutively and not repressed by the presence of glucose or absence of lactose. In other words, lacI expression is not repressed under these conditions, making this statement incorrect.

Option 5: 'The lac repressor protein is bound to the operator.' — This is exactly what happens when lactose is absent: the repressor binds the operator and blocks transcription. This statement is true.

In summary, options 1, 2, 3, and 5 are consistent with glucose present and lactose absent, while option 4 contradicts the typical regulation of lacI expression under these conditions.

When glucose is present, and lactose is absent in the bacteria cell’s environment, which of the following is INCORRECT with respect to the lac operon?单项选择题

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