Question:
During the expression of Lac operon, the repressor protein binds to
During the expression of Lac operon, the repressor protein binds to
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The repressor protein acts as a physical roadblock.
By binding to the operator, it stops RNA polymerase from transcribing the structural genes.
By binding to the operator, it stops RNA polymerase from transcribing the structural genes.
Updated On: Jul 22, 2026
- Promoter
- Operator
- Inducer
- Terminator
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The Correct Option is B
Solution and Explanation
Step 1: Understanding the Question:
The question asks to identify the specific DNA sequence within the lac operon where the active repressor protein binds to regulate gene transcription.
Step 2: Key Concepts and Approach:
The lac operon is a classic genetic system in bacteria (E. coli) that regulates lactose metabolism.
It consists of structural genes (lacZ, lacY, lacA) and regulatory regions (promoter, operator, regulator gene \(i\)).
Step 3: Detailed Explanation:
• Regulator Gene (\(i\)): This gene codes for the lac repressor protein.
The repressor is synthesized constitutively in an active form.
• Operator Region (O): The active repressor protein binds specifically to the operator DNA sequence.
When the repressor is bound to the operator, it physically blocks RNA polymerase from binding to the adjacent promoter or moving down the operon.
This prevents transcription of the structural genes (negative regulation).
• Role of the Inducer (Lactose/Allolactose): In the presence of an inducer, the inducer binds to the repressor protein.
This inactivates the repressor, changing its shape so it can no longer bind to the operator.
This clears the path for RNA polymerase to transcribe the structural genes.
Step 4: Final Answer:
The repressor protein binds directly to the operator region to block transcription.
The question asks to identify the specific DNA sequence within the lac operon where the active repressor protein binds to regulate gene transcription.
Step 2: Key Concepts and Approach:
The lac operon is a classic genetic system in bacteria (E. coli) that regulates lactose metabolism.
It consists of structural genes (lacZ, lacY, lacA) and regulatory regions (promoter, operator, regulator gene \(i\)).
Step 3: Detailed Explanation:
• Regulator Gene (\(i\)): This gene codes for the lac repressor protein.
The repressor is synthesized constitutively in an active form.
• Operator Region (O): The active repressor protein binds specifically to the operator DNA sequence.
When the repressor is bound to the operator, it physically blocks RNA polymerase from binding to the adjacent promoter or moving down the operon.
This prevents transcription of the structural genes (negative regulation).
• Role of the Inducer (Lactose/Allolactose): In the presence of an inducer, the inducer binds to the repressor protein.
This inactivates the repressor, changing its shape so it can no longer bind to the operator.
This clears the path for RNA polymerase to transcribe the structural genes.
Step 4: Final Answer:
The repressor protein binds directly to the operator region to block transcription.
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