Question

In lac operon, the regulatory protein called repressor binds to :

• A. Promoter
• B. Operator
• C. Enhancer
• D. Regulator

Hint:

Remember that the promoter is the binding site for RNA polymerase, while the operator is the specific DNA sequence bound by the repressor protein.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The lac operon of Escherichia coli is a classic model of bacterial gene regulation.
It is an inducible system regulated by the accessibility of RNA polymerase to the promoter region, which is controlled by a repressor protein.

Step 2: Detailed Explanation:
The operon consists of structural genes (lacZ, lacY, and lacA), a promoter, an operator, and a regulator gene (lacI) that codes for the lac repressor.
Under normal conditions when lactose is absent, the active tetrameric repressor protein, synthesized by the regulator gene, binds with high affinity to the operator site.
The operator site is situated downstream of the promoter and overlaps with the transcription start site.
Physical binding of the repressor to the operator sterically blocks RNA polymerase from transcribing the structural genes, keeping the operon repressed.
When lactose is present, its isomer allolactose acts as an inducer, binding to the repressor, changing its conformation, and preventing it from binding to the operator, which allows transcription to proceed.

Step 3: Final Answer:
Hence, the regulatory repressor protein binds to the Operator.