Q.76 In an experiment the structural genes lacZYAof lac operon were found to be constitutively
expressed. The following explanations were given for the constitutive expression
(P) absence of a functional repressor due to mutation in the repressor gene lacI
(Q) mutation in the operator that can no longer bind the repressor
(R) mutation in the lacAgene
Which of the following is CORRECT
(A) Only P (B) Only Q (C) Both P & Q (D) Only R
Constitutive expression of lacZYA occurs when repression fails, either due to non-functional lacI repressor (P) or operator mutation preventing repressor binding (Q). lacA mutation (R) affects only transacetylase function, not regulation. The correct answer is (C) Both P & Q.
Correct Answer
(C) Both P & Q
Lac Operon Regulation Basics
Wild-type lac operon: lacI encodes repressor that binds operator (lacO), blocking RNA polymerase at promoter. Lactose → allolactose binds repressor, releasing it for inducible expression. Constitutive = always ON regardless of lactose.
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P (lacI⁻ mutation): Defective repressor can’t bind operator → continuous transcription.
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Q (Oᶜ mutation): Operator can’t bind repressor → continuous transcription.
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R (lacA mutation): Structural gene defect; regulation intact, just no functional transacetylase.
Option Analysis
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(A) Only P: Wrong. Ignores operator mutations (equally common constitutive cause).
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(B) Only Q: Wrong. Excludes lacI⁻ mutants (classic Jacob/Monod discovery).
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(C) Both P & Q: Correct. Both disrupt repressor-operator interaction.
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(D) Only R: Wrong. lacA is downstream effector; doesn’t control expression.
Introduction to Lac Operon Constitutive Expression
Lac operon constitutive expression lacI operator mutation defines GATE Life Sciences molecular biology Q.76, testing why lacZYA express without lactose. Both lacI⁻ (no repressor) and Oᶜ (unbindable operator) cause continuous β-galactosidase/permease production.
Constitutive Mutation Mechanisms
Wild-type: lacI --|--> [P-O-ZYA] (repressed)
lacI⁻ (P): X [P-O-ZYA] (constitutive)
Oᶜ (Q): lacI --|--> [P-Ø-ZYA] (constitutive)
lacA (R): lacI --|--> [P-O-ZYø] (inducible, defective product)
P/Q bypass repression; R doesn’t affect control.
Q.76 Elimination Table
| Explanation | Affects Repression? | Constitutive? |
|---|---|---|
| P: lacI mutation | ✅ No repressor | ✅ Yes |
| Q: Operator mutation | ✅ No binding | ✅ Yes |
| R: lacA mutation | ❌ Structural only | ❌ No |
| Answer: (C) P & Q |
Classic Mutant Types
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lacI⁻: Trans-acting (affects all operon copies).
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Oᶜ: Cis-acting (affects only adjacent operon).
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Superrepressor Iˢ: Always binds, even with inducer.
GATE Molecular Biology Tips
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Mnemonic: “I⁻ and Oᶜ = Always ON.”
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Distinguish: Constitutive (always) vs uninducible (never).
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PYQ trap: lacA mutation = function loss, not regulation loss.
Master lac operon constitutive expression lacI operator mutation for perfect genetics scores in competitive exams.
