Question

What is the function of the substance labelled A in this diagram of the Lac operon?

Hint:

The Lac operon is an example of "negative regulation" because the default state is inhibited by a repressor. The inducer removes this inhibition.

Answer 1

Step 1: Understanding the Concept:
The Lac operon is a system for the regulated expression of genes involved in lactose metabolism in E. coli.
Step 2: Detailed Explanation: Substance A is the Inducer.
In the absence of an inducer, the repressor protein (produced by the 'i' gene) binds to the operator region and prevents RNA polymerase from transcribing the structural genes.
When the inducer (Lactose) is present, it binds to the repressor.
This binding causes a conformational change in the repressor, making it inactive and unable to bind to the operator.
This allows RNA polymerase access to the promoter, and transcription proceeds (the operon is switched "ON").
Step 3: Final Answer:
The function of substance A is to inactivate the repressor so that the operon can be transcribed.