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Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

Maharashtra BoardEnglish MediumSTD 12 ScienceBIOLOGYMolecular Basis of Inheritance1 Mark
MCQ
For question, two statements are given$-$one labelled Assertion and the other labelled Reason. Select the correct answer to these questions from the codes $(a), (b), (c)$ and $(d)$ as given below.
Assertion: No lac $\text{mRNA}$ is made in the presence of glucose.
Reason: In the presence of glucose and lactose activity of lac operon is not needed.
  • ✓
    Both assertion and reason are true, and reason is the correct explanation of assertion.
  • B
    Both assertion and reason are true, but reason is not the correct explanation of assertion.
  • C
    Assertion is true, but reason is false.
  • D
    Both assertion and reason are false
✓

Answer

Correct option: A.
Both assertion and reason are true, and reason is the correct explanation of assertion.
The function of $\beta$ galactosidase enzyme in lactose metabolism is to form glucose by cleaving lactose. Thus if both glucose and lactose are present in the growth medium, activity of lac operon is not needed, and indeed, no $\beta$ galactosidase is formed until virtually all of the glucose in the culture medium is consumed. The lack of synthesis of $\beta$ galactosidase is a result of lack of synthesis of lac $\text{mRNA}.$ No lac $\text{mRNA}$ is made in the presence of glucose, because in addition of an inducer to inactivate the lac i repressor, another element $\text{(cAMP-CAP)}$ is needed for initiating lac $\text{mRNA}$ synthesis, the activity of this element is regulated by the concentration of glucose. However, the inhibitory effect of glucose on expression of lac operon is quite indirect.

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