Q.29 Determine the correctness or otherwise of the following Assertion [a] and the Reason [r].
Assertion [a]: lac operon is an inducible operon.
Reason [r]: lac operon is not induced when the repressor protein remains bound to operator
DNA sequence.
(A) Both [a] and [r] are true and [r] is the correct reason for [a]
(B) Both [a] and [r] are true but [r] is not the correct reason for [a]
(C) Both [a] and [r] are false
(D) [a] is true but [r] is false

Correct Answer: (A) Both [a] and [r] are true and [r] is the correct reason for [a]

The lac operon is induced only when lactose (via allolactose) binds the repressor, causing it to release from the operator and allow transcription.​

Lac Operon Basics

The lac operon in E. coli regulates lactose metabolism genes (lacZ, lacY, lacA) through negative control. Without lactose, the lac repressor (from lacI) binds the operator, blocking RNA polymerase access to the promoter. Lactose presence converts to allolactose, an inducer that alters repressor conformation, detaching it from the operator for transcription.​

Option Analysis

• (A) Both true, [r] explains [a]: Assertion [a] holds as the lac operon requires inducer for activation, unlike repressible operons (e.g., trp). Reason [r] directly explains this: repressor-operator binding prevents induction, so removal enables it—making [r] the precise mechanism.​

• (B) Both true, but [r] not reason: Incorrect; [r] is the core inducible mechanism, not unrelated.​

• (C) Both false: Wrong; both statements match established regulation.​

• (D) [a] true, [r] false: No; [r] accurately describes repression blocking induction.​

The lac operon inducible mechanism allows E. coli to efficiently metabolize lactose only when needed, a key CSIR NET Life Sciences topic. This inducible operon stays off via repressor-operator binding until lactose triggers induction.​

Key Components

• Structural genes: lacZ (β-galactosidase), lacY (permease), lacA (transacetylase).​

• Regulatory elements: Promoter, operator (O1, binding site for repressor), lacI (repressor gene).​

• Induction process: Allolactose binds repressor, causing allosteric change and operator release.​

Repressor-Operator Interaction

Lac repressor (tetramer) binds operator tightly in no-lactose conditions, blocking transcription—ensuring no wasteful enzyme production. This binding affinity drops 1000-fold with inducer, enabling RNA polymerase access.​

Why Inducible?

Unlike repressible operons (corepressor activates repressor), lac operon uses inducer to inactivate repressor, ideal for catabolizing rare substrates like lactose.​

CSIR NET Relevance

Assertion-reason questions test this: lac operon induction fails with repressor bound to operator, confirming option (A). Glucose catabolite repression (via CAP-cAMP) adds complexity but doesn’t alter basic inducibility.​​