lac Oc Mutation is Cis-Dominant

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January 31, 2026
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Q79. In a lacO^c lacZ⁻ / lacO⁺ lacZ⁺ partial diploid, of the two lacZ enzymes, only the mutant enzyme (lacZ⁻) is synthesized constitutively. This observation shows that lacO^c mutation is

trans-dominant
trans-recessive
cis-dominant
cis-recessive

✅ Correct Answer: C. cis-dominant

Introduction

The lac operon is a classic example of gene regulation in prokaryotes. It controls lactose metabolism through coordinated expression of structural genes regulated by an operator and repressor system.

Structure of lac Operon

lacZ → β-galactosidase
lacY → permease
lacA → transacetylase
Operator (O) → repressor binding site
Promoter (P) → RNA polymerase binding site

Concept Behind lacO^c Mutation

Step 1: Meaning of lacO^c

The operator mutation prevents binding of the lac repressor. Therefore, transcription occurs continuously (constitutive expression).

Step 2: Cis vs Trans Effects

Cis: acts only on the same DNA molecule
Trans: acts through diffusible products like proteins

Step 3: Interpretation of Given Genotype

Only lacZ⁻ linked to lacO^c shows constitutive expression, while lacZ⁺ linked to lacO⁺ remains regulated. Hence, the mutation affects only adjacent genes.

Explanation of All Options

Option A: trans-dominant ❌

Trans-dominant mutations produce diffusible products that affect both operons. The operator is a DNA sequence, not a protein, so this is incorrect.

Option B: trans-recessive ❌

Trans-recessive mutations require diffusible gene products and recessive behavior. lacO^c is neither diffusible nor recessive.

Option C: cis-dominant ✅

Operator mutations act locally on the same DNA molecule and show dominant constitutive expression. Therefore, lacO^c is cis-dominant.

Option D: cis-recessive ❌

If recessive, wild-type operator would mask the mutation. Since constitutive expression still occurs, it is dominant, not recessive.

Quick Comparison for Exams

Mutation	Type	Nature
lacI⁻	Trans	Diffusible repressor protein
lacO^c	Cis	Local DNA site only

Memory Trick

Operator = On-site control → Always cis acting

Final Answer

C. cis-dominant

Q79. In a lacO^c lacZ⁻ / lacO⁺ lacZ⁺ partial diploid, of the two lacZ enzymes, only the mutant enzyme (lacZ⁻) is synthesized constitutively. This observation shows that lacO^c mutation is

✅ Correct Answer: C. cis-dominant

Introduction

Structure of lac Operon