Which of the following events would occur in 'Lac-operon' of E.Coli when the growth medium has high concentration of lactose?

The structural genes fail to produce polycistronic mRNA.

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The repressor protein binds to RNA polymerase and prevents translation.

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The repressor protein attaches to the promoter sequence and derepresses the operator.

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The inducer molecule binds to repressor protein and RNA polymerase binds to promoter sequence.

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Solution

The correct option is D The inducer molecule binds to repressor protein and RNA polymerase binds to promoter sequence.
Lactose (or allolactose) acts as an inducer in 'Lac' operon model. When lactose is present in high concentration in growth medium, it binds to repressor protein, which then fails to bind to operator gene. This results in RNA polymerase binding to promoter region in gene and transcription of structural genes.
Thus, the correct answer is option B.

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\begin{matrix} C o l u m n I & C o l u m n I I p . R e g u l a t o r g e n e & i . B i n d i n g s i t e f o r r e p r e s s o r p r o t e i n q . O p e r a t o r g e n e & i i . C o d e (s) f o r t h e p r o t e i n o f t h e e n z y m e s r . P r o m o t e r g e n e & i i i . C o d e (s) f o r r e p r e s s o r p r o t e i n s . S t r u c t u r a l g e n e s & i v . B i n d i n g S i t e f o r R N A p o l y m e r a s e \end{matrix}

Column-I	Column-II
(A) Structural gene	(i) Binding site for a repressor protein
(B) Operator gene	(ii) Codes for a repressor protein
(C) Promoter gene	(iii) Induce lactose transport from the medium
(D) Regulator gene	(iv) Codes for protein enzyme
	(v) Binding site for RNA polymerase

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