Question

Given below are two statements, one is labelled as Assertion (A) and other labelled as Reason (R).
• [Assertion (A):] During DNA replication, the lagging strand is replicated in short pieces. 
• [Reason (R):] DNA polymerase can synthesize the DNA only in one direction i.e. 5' to 3'. 
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:

Remember:
- Leading strand: Synthesized continuously toward the replication fork.
- Lagging strand: Synthesized discontinuously as Okazaki fragments away from the replication fork.
This asymmetry is caused by the 5' to 3' directional limitation of DNA polymerase.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
DNA replication is a highly coordinated process.
Because the two strands of a DNA double helix are antiparallel, replication must proceed in opposite directions on each template strand.

Step 3: Detailed Explanation:
Let us evaluate the Assertion and Reason statements:
- Assertion (A) is true: During replication, the lagging strand is synthesized discontinuously in short segments called Okazaki fragments.
These fragments are later joined together by DNA ligase to form a continuous strand.
- Reason (R) is true: DNA polymerase enzymes can only add new nucleotides to the free 3'-OH group of an existing primer or strand.
This restricts DNA synthesis to the 5' to 3' direction.
- Evaluating the connection: The replication fork opens in one direction.
Because DNA synthesis can only occur in the 5' to 3' direction, only the leading strand can be synthesized continuously in the direction of fork movement.
The other template strand (the lagging strand) runs in the opposite direction.
To synthesize this strand in the 5' to 3' direction, DNA polymerase must move away from the replication fork.
As the fork opens, the enzyme must repeatedly restart synthesis, producing short, discontinuous Okazaki fragments.
Therefore, the directional restriction of DNA polymerase (Reason) explains why the lagging strand is synthesized in short pieces (Assertion).

Step 4: Final Answer:
Both (A) and (R) are true, and (R) is the correct explanation of (A), matching Option (A).