Question at position 9 When both glucose and lactose are present in the growth media the lac operon is only active at low levels. This is because:The presence of glucose increase cAMP levels which is a co-repressor for CRP protein. cAMP bound CRP binds the lac operator to repress transcription of the operon.The presence of glucose reduces cAMP levels, which is an inducer for the CRP repressor. CRP is not bound to cAMP and can bind the operator to prevent transcription of an operon.The presence of glucose increases cAMP levels causing an allosteric change in CRP that prevents it from binding DNA and activating transcription.The presence of glucose reduces cAMP levels that is required for the positive regulator, CRP to bind DNA and activate transcription of the operon.There is a mutation in the promoter that prevents activation of the operon.

Question

Question at position 9 When both glucose and lactose are present in the growth media the lac operon is only active at low levels. This is because:The presence of glucose increase cAMP levels which is a co-repressor for CRP protein. cAMP bound CRP binds the lac operator to repress transcription of the operon.The presence of glucose reduces cAMP levels, which is an inducer for the CRP repressor. CRP is not bound to cAMP and can bind the operator to prevent transcription of an operon.The presence of glucose increases cAMP levels causing an allosteric change in CRP that prevents it from binding DNA and activating transcription.The presence of glucose reduces cAMP levels that is required for the positive regulator, CRP to bind DNA and activate transcription of the operon.There is a mutation in the promoter that prevents activation of the operon.

BlackTom AI · Accepted Answer

Re-stating the core scenario helps frame the analysis: when both glucose and lactose are present, lac operon activity is kept at a low level due to catabolite repression via CRP-cAMP dynamics.
Option 1: The presence of glucose increase cAMP levels which is a co-repressor for CRP protein. cAMP bound CRP binds the lac operator to repress transcription of the operon. 
This is incorrect. Glucose actually lowers, not raises, cAMP levels in the cell. Moreover, CRP bound to cAMP acts as a positive regulator that helps activate transcription by assisting RNA polymerase binding; it does not repress by binding the lac operator alone. So this statement misstates both the cAMP change and the regulatory role of CRP-cAMP.
Option 2: The presence of glucose reduces cAMP levels, which is an inducer for the CRP repressor. CRP is not bound to cAMP and can bind the operator to prevent transcription of an operon. 
This is incorrect. cAMP bound CRP is the active form that stimulates transcription by helping RNA polymerase bind the promoter. When cAMP is low (as with glucose present), CRP is not activating transcription. The idea that CRP acts as a repressor when not bound to cAMP contradicts the established model where CRP-cAMP is the activator, not a repressor, in this operon context.
Option 3: The presence of glucose increases cAMP levels causing an allosteric change in CRP that prevents it from binding DNA and activating transcription. 
This is incorrect. Glucose does not increase cAMP levels; it decreases them. Moreover, the CRP-cAMP complex is the activator that binds DNA to promote transcription; an allosteric change that prevents binding is not consistent with the known mechanism of CRP as a cAMP-dependent activator in the lac operon.
Option 4: The presence of glucose reduces cAMP levels that is required for the positive regulator, CRP to bind DNA and activate transcription of the operon. 
This statement aligns with the classic model. Lower cAMP levels mean less CRP-cAMP complex, so CRP is less able to facilitate transcription initiation, resulting in only low-level expression when lactose is present to partially relieve repression. It describes the correct regulatory interplay between glucose (catabolite repression) and lactose (induction).
Option 5: There is a mutation in the promoter that prevents activation of the operon. 
This introduces an extrinsic genetic factor not indicated by the scenario. The question describes a physiological condition (glucose presence) affecting regulation via CRP-cAMP, not a promoter mutation, so this option is not consistent with the given context.

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