LABORATORY REVIEW
MOLECULAR BIOLOGY
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Question
[CLICK ON ANY CHOICE TO KNOW THE RIGHT ANSWER]
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Glucose present, lactose absent
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Glucose present, lactose present
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Glucose absent, lactose absent
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Glucose absent, lactose present
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Detailed explanation-1: -Glucose absent, lac tose present: Strong transcription of the lac operon occurs. The lac repressor is released from the operator because the inducer (allolactose) is present. cAMP levels are high because glucose is absent, so CAP is active and bound to the DNA.
Detailed explanation-2: -Second, lactose must be present. If glucose is absent, then CAP can bind to the operator sequence to activate transcription. If lactose is absent, then the repressor binds to the operator to prevent transcription. If either of these requirements is met, then transcription remains off.
Detailed explanation-3: -However, in the absence of glucose, the lac operon can be activated if the lac i repressor is turned off by lactose binding. More specifically, in E. coli, the switch from glucose use to lactose use depends on the presence of both cAMP and a molecule called catabolite activator protein (CAP).
Detailed explanation-4: -When lactose is absent the lac operon is switched off. This is becasue a repressor protein is produced which binds to the operator region. This prevetns RNA polymerase from binding to the operon and therefore prevents transcription of the structual genes.