Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: The mode of DNA replication is semi-conservative in nature.
Reason R: One of the DNA strands is synthesized continuously, while the other is synthesized discontinuously.

In light of the above statements, choose the most appropriate answer from the options given below:

• A. Both A and R are correct and R is the correct explanation of A
• B. Both A and R are correct but R is NOT the correct explanation of A
• C. A is correct but R is not correct
• D. A is not correct but R is correct

Hint:

- Semi-conservative replication: Proven by Meselson and Stahl using $N^{15}$ isotope. Each daughter molecule has one old and one new strand.
- Semi-discontinuous replication: Consists of a continuous leading strand and a discontinuous lagging strand.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
DNA replication involves unwinding the parental double helix and synthesizing complementary strands.
Replication exhibits both semi-conservative and semi-discontinuous properties.

Step 2: Detailed Explanation:
Let us analyze both statements:
Assertion A:
The mode of DNA replication is semi-conservative.
This means that during replication, the two strands of the parental DNA double helix separate, and each acts as a template for the synthesis of a new complementary strand.
Each resulting daughter DNA molecule contains one original (conserved) parental strand and one newly synthesized strand.
Thus, Assertion A is correct.
Reason R:
Because DNA polymerase can only synthesize DNA in the 5' to 3' direction, and the two template strands are antiparallel:
- One strand (the leading strand) is synthesized continuously toward the replication fork.
- The other strand (the lagging strand) is synthesized discontinuously away from the fork, forming Okazaki fragments.
This describes the semi-discontinuous nature of DNA replication.
Thus, Reason R is correct.
Analyzing the connection:
While both statements describe properties of DNA replication, the semi-discontinuous nature of synthesis (Reason R) is a consequence of the unidirectional activity of DNA polymerase and the antiparallel nature of DNA.
It does not explain why replication is semi-conservative (which is a consequence of template-directed complementary base pairing).
Therefore, both statements are correct, but R is not the correct explanation of A.

Step 3: Final Answer:
Both A and R are correct, but R is NOT the correct explanation of A.
This corresponds to Option (B).