Question

In the lac operon, which molecule acts as the inducer to initiate transcription?

• A. Lactose
• B. Allolactose
• C. Glucose
• D. Galactose

Hint:

{Lac Operon Inducer = Allolactose}.
It binds the lac repressor and allows transcription.

Answer 1

The Correct Option is B

Concept: The lac operon is a gene regulatory system found in {E. coli} that controls the metabolism of lactose. It is an example of an inducible operon, meaning that transcription is normally off but can be activated in the presence of an inducer molecule.
Step 1:Structure of the lac operon.
The lac operon consists of:

• Structural genes: (lacZ), (lacY), (lacA)
• Promoter and operator regions
• A regulatory gene producing the lac repressor

Step 2:Role of the lac repressor.
In the absence of lactose, the lac repressor protein binds to the operator region and blocks RNA polymerase from transcribing the structural genes.
Step 3:Action of the inducer molecule.
When lactose enters the cell, a small portion of it is converted into allolactose. Allolactose binds to the lac repressor and changes its shape, preventing it from binding to the operator.
Step 4:Initiation of transcription.
Once the repressor is removed from the operator, RNA polymerase can bind to the promoter and initiate transcription of the lac operon genes. Thus, the molecule that acts as the true inducer of the lac operon is allolactose.