Question

What would happen if the substance labelled A was not present?

Hint:

Regulation by a repressor is called negative regulation. In the absence of lactose, the repressor is the "brake" of the system.

Answer 1

Step 1: Understanding the Concept:
The regulation of the Lac operon is an energy-saving mechanism; the cell only produces enzymes when the substrate is available.
Step 2: Detailed Explanation:
If the inducer (lactose) is not present:
1. The 'i' gene constitutively synthesizes the repressor mRNA, which is translated into an active repressor protein.
2. This active repressor binds firmly to the operator region (o) of the operon.
3. The bound repressor physically blocks the RNA polymerase from moving forward from the promoter to the structural genes.
4. Consequently, transcription of \( z, y, \text{ and } a \) genes does not occur, and no enzymes are produced.
Step 3: Final Answer:
The operon would remain in a repressed (OFF) state, and no enzymes for lactose metabolism would be synthesized.