Sample 1Medium
A diagram of the lac operon of E. coli shows, in order, the i gene, then p, o, z, y and a. In panel 1 the protein R, made from the i gene, is bound to o and no transcription occurs. In panel 2 an inducer molecule binds R, R leaves o, and RNA polymerase transcribes z, y and a. Which statement is correct?
- A
R is the repressor made constitutively from the i gene, and lactose acts as the inducer that inactivates it
- B
R is RNA polymerase, and the i gene is switched on only by lactose
- C
z codes for permease and y codes for beta-galactosidase
- D
Glucose is the inducer that binds R
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The answer is A.The i gene codes for the repressor, which binds the operator and prevents transcription. Lactose, the inducer, binds and inactivates the repressor so that RNA polymerase can transcribe z (beta-galactosidase), y (permease) and a (transacetylase).
- B.
The i gene codes for the repressor and is expressed constitutively.
- C.
z codes for beta-galactosidase and y for permease.
- D.
The inducer of the lac operon is lactose, not glucose.
NCERT: Class 12 Biology, Chapter 5, 5.8
Sample 2NEET level
Assertion (A): Lactose acts as the inducer of the lac operon in E. coli. Reason (R): The y gene of the lac operon codes for beta-galactosidase.
- A
Both A and R are true and R is the correct explanation of A
- B
Both A and R are true but R is not the correct explanation of A
- C
A is true but R is false
- D
A is false but R is true
Show the answer
The answer is C.Lactose (the substrate of beta-galactosidase) is the inducer: it binds the repressor, inactivates it and allows RNA polymerase to transcribe the operon, so A is true. The z gene codes for beta-galactosidase, while the y gene codes for permease, which increases the permeability of the cell to beta-galactosides, so R is false.
- A.
R is false: beta-galactosidase is coded by the z gene, not the y gene.
- B.
R is false, so it cannot be a true statement unrelated to A.
- D.
A is true: lactose inactivates the repressor and switches the operon on.
NCERT: Class 12 Biology, Chapter 5 (Molecular Basis of Inheritance), Regulation of gene expression: the lac operon