Question

Determine the correctness or otherwise of the following Assertion [a] and the Reason [r].
Assertion [a]: lac operon is an inducible operon.
Reason [r]: lac operon is not induced when the repressor protein remains bound to operator DNA sequence.

• A. Both [a] and [r] are true and [r] is the correct reason for [a]
• B. Both [a] and [r] are true but [r] is not the correct reason for [a]
• C. Both [a] and [r] are false
• D. [a] is true but [r] is false

Hint:

In inducible operons like the lac operon, the presence of an inducer (lactose) binds to the repressor, causing it to release from the operator and allowing gene expression.

Answer 1

The Correct Option is B

The given question involves understanding the functioning of the lac operon and involves determining the accuracy of both the assertion and the reason provided. Let's break down the question step-by-step.

1. Understanding the Assertion:

   Assertion [a]: lac operon is an inducible operon.

   An inducible operon is one that is usually in an "off" state, but can be turned "on" by an inducer. The lac operon in E. coli is a classic example of an inducible operon. In the absence of lactose, a repressor protein binds to the operator region of the operon, preventing transcription. When lactose is present, it acts as an inducer by binding to the repressor protein, causing it to change shape and release from the operator, allowing transcription to proceed. Hence, the assertion that the lac operon is inducible is true.

2. Understanding the Reason:

   Reason [r]: lac operon is not induced when the repressor protein remains bound to the operator DNA sequence.

   This is a correct statement. The function of the repressor is to prevent the expression of the genes in the operon by binding to the operator region. When the repressor is bound, the RNA polymerase cannot transcribe the downstream genes required for lactose metabolism. Therefore, the operon remains "off," confirming that this reason is true.

3. Analyzing the Relationship Between Assertion and Reason:

   Both the assertion and the reason are statements about the lac operon, and both are true. However, the reason does not directly explain why the lac operon is classified as an inducible operon. Instead, it explains a mechanism of how the operon is kept "off" in the absence of the inducer (lactose). Therefore, the reason, while true, is not the correct explanation for the assertion being true.

4. Conclusion:

   The correct answer is: Both [a] and [r] are true but [r] is not the correct reason for [a].