Question

In the case of Lac operon, the presence of glucose in the cell/growing media will lead to

• A. Increased induction
• B. Catabolite repression
• C. Slow induction
• D. Zero effect induction

Hint:

Think of catabolite repression as an energy-saving switch: the cell will not synthesize enzymes to digest lactose as long as its preferred energy source, glucose, is available.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The lac operon in Escherichia coli regulates the transport and metabolism of lactose.
Because glucose is a more energetically efficient carbon source, E. coli will repress the lac operon if glucose is available, even in the presence of lactose.

Step 3: Detailed Explanation:
This inhibitory control is called catabolite repression (or the glucose effect).
It is mediated by cyclic AMP (cAMP) and the Catabolite Activator Protein (CAP), which acts as a positive transcription factor:
- When glucose levels are high, the rate of glycolysis is high, which inhibits the enzyme adenylate cyclase.
- This inhibition causes intracellular levels of cAMP to drop.
- Without cAMP, CAP cannot form the cAMP-CAP complex required to bind to the CAP-binding site upstream of the lac promoter.
- Without CAP bound, RNA polymerase cannot bind to the promoter efficiently, resulting in very low levels of transcription, even if lactose is present.
Once glucose is depleted, adenylate cyclase is activated, cAMP levels rise, and the cAMP-CAP complex binds to activate high-level transcription of the operon.

Step 4: Final Answer:
The presence of glucose in the growth medium results in catabolite repression of the lac operon, matching Option (B).