Question

When Escherichia coli cells are cultured in a medium where Lactose is absent, the 'i' gene of Lac Operon continues to produce repressor mRNA, because it is

• A. a non-coding gene
• B. an operator gene
• C. a constitutive gene
• D. a structural gene

Answer 1

The Correct Option is C

The 'i' gene (regulator gene) of the Lac Operon encodes the lac repressor protein. It is a constitutive gene, meaning it is continuously expressed (transcribed into mRNA) regardless of the presence or absence of lactose. In the absence of lactose, the repressor binds to the operator, blocking transcription of the structural genes (lacZ, lacY, lacA). In the presence of lactose (inducer), lactose binds to the repressor, preventing it from blocking the operator, allowing gene expression.

• (A) Non-coding gene: Incorrect because the 'i' gene produces functional repressor mRNA.
• (B) Operator gene: Incorrect because the operator is a DNA binding site, not a gene.
• (D) Structural gene: Incorrect because structural genes (lacZ, lacY, lacA) are regulated, not constitutively expressed.

Answer 2

The 'i' gene of the Lac operon produces the repressor protein continuously, regardless of lactose presence.
Constitutive genes (like 'i') are always active, ensuring baseline repressor production.
When lactose is absent, the repressor binds to the operator, blocking transcription of structural genes (lacZ, lacY, lacA).

Why other options are incorrect:

• (A) Non-coding gene: The 'i' gene codes for the repressor protein.
• (B) Operator gene: It is a DNA binding site, not a gene.
• (D) Structural gene: The 'i' gene is regulatory, not structural.

Correct Answer: (C) A constitutive gene