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 absent, lactose absent
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Glucose present, lactose present
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Glucose absent, lactose present
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Detailed explanation-1: -Glucose present, lac tose present: Low-level transcription of the lac operon occurs. The lac repressor is released from the operator because the inducer (allolactose) is present. cAMP levels, however, are low because glucose is present.
Detailed explanation-2: -If both glucose and lactose are both present, lactose binds to the repressor and prevents it from binding to the operator region. The block of lac gene transcription is thus lifted, and a small amount of mRNA is produced.
Detailed explanation-3: -Figure 1: The lac operon is activated when intracellular glucose levels are low. When the concentration of intracellular glucose is low (upper panel), the levels of the signal molecule cAMP are high. cAMP readily binds with CAP, and the CAP-cAMP complex binds DNA at a position upstream of the lac operon.
Detailed explanation-4: -In the absence of lactose the lac repressor blocks the expression of the lac operon by binding to the DNA at a site, called the operator that is downstream of the promoter and upstream of the transcriptional initiation site.
Detailed explanation-5: -Transcription of the E. coli lac operon in a lactose-containing medium is strongly repressed by the presence of glucose, resulting in a diauxic growth curve.