Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A : DNA polymerase can synthesize DNA in the 5' to 3' direction.
Reason R : DNA polymerase can synthesize DNA in the 3' to 5' direction.

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

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

Hint:

Keep this rule in mind:
- Synthesis of new DNA strand: Always 5' to 3' direction.
- Reading of template DNA strand: Always 3' to 5' direction.
This directional requirement is why lagging strand synthesis is discontinuous (forming Okazaki fragments).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
DNA replication is catalyzed by the enzyme DNA polymerase.
The biochemical mechanism of DNA synthesis imposes strict directional constraints on the enzyme's activity.

Step 2: Detailed Explanation:
Let us analyze both statements:
Assertion A:
DNA polymerases catalyze the formation of a phosphodiester bond between the 3'-OH group of the existing deoxyribose sugar and the 5'-phosphate group of an incoming dNTP.
Because they require a free 3'-OH group to attach the next nucleotide, the synthesis of a new DNA strand can occur exclusively in the 5' to 3' direction.
Thus, Assertion A is true.
Reason R:
Since DNA polymerase lacks the biochemical mechanism to add nucleotides to a free 5'-phosphate end during synthesis, it cannot synthesize DNA in the 3' to 5' direction.
Although DNA polymerase can move along the template strand in the 3' to 5' direction, the newly synthesized strand is made in the 5' to 3' direction.
Thus, Reason R is false.

Step 3: Final Answer:
Assertion A is true, but Reason R is false.
Therefore, the correct option is (C).