Question

Gene regulation governing lactose operon of $E$. $coli$ that involves the $lac$ $i$ gene products is

• A. Negative and inducible because repressor protein prevents transcription
• B. Negatively and repressible because repressor protein prevents transcription
• C. Feedback inhibition because excess of b-galactosidase can switch off transcription
• D. Positive and inducible because it can be induced by lactose

Answer 1

The correct option is A Negative and inducible because repressor protein prevents transcription

The $lac$ operon
The $lac$ operon is an inducible operon system that remains usually in a repressed condition and gets activated only when the substance to be metabolised (lactose) is present.
Negative regulation of $lac$ operon
A $lac$ operon has a regulator gene ($i$-inhibitor gene) which encodes for a repressor protein.
Normally, the repressor protein binds to the operator region, blocking the RNA polymerase and preventing the transcription. This is because the RNA polymerase cannot bind properly to the promoter which has overlaps with the operator region. Hence, the transcription of the downstream structural genes don’t take place. The lac operon is switched off. This is negative regulation.
Final answer
(D) Negative and inducible because repressor protein prevents transcription