GENETICS AND EVOLUTION
MOLECULAR BASIS OF INHERITANCE
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Question
[CLICK ON ANY CHOICE TO KNOW THE RIGHT ANSWER]
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there is glucose but no lactose in the cell.
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the cyclic AMP levels are low.
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the cyclic AMP and lactose levels are both high within the cell.
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there is more glucose in the cell than lactose.
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Detailed explanation-1: -Answer: C. The cyclic AMP and lactose levels are both high within the cell The lac operon process undergoes repression by the lac repressor .
Detailed explanation-2: -CAP is only active when glucose levels are low (cAMP levels are high). Thus, the lac operon can only be transcribed at high levels when glucose is absent.
Detailed explanation-3: -Transcription of the lac operon normally occurs only when lactose is available for it to digest. Presumably, this avoids wasting energy in the synthesis of enzymes for which no substrate is present. A single mRNA transcript includes all three enzyme-coding sequences and is called polycistronic.
Detailed explanation-4: -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-5: -In the absence of lactose in the environment, the lac operon is transcribed at very low levels (Figure 2). However, when lactose appears in the environment, a molecule produced from it (allolactose) can bind to the repressor (lacI protein), thereby causing a conformational change.