Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : The use of RNA primer eliminates the source of error during replication.
Reason (R) : RNA is eventually replaced by DNA that permits actual base pairing to be achieved.
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:

If DNA replication could start *de novo*, the highly error-prone first few nucleotides would be permanently incorporated into the genome.
Using an RNA primer acts as a "temporary scaffold" that is easily identified, removed, and replaced with high-fidelity DNA.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
DNA replication must occur with extremely high fidelity to prevent mutations.
However, DNA polymerases cannot initiate the synthesis of a new DNA strand *de novo*; they require a pre-existing 3'-OH group provided by a primer.
Detailed Explanation:
- Evaluating Assertion (A): During replication initiation, the enzyme primase synthesizes a short RNA primer.
Because the initiation of a polynucleotide chain is inherently less accurate and cannot undergo standard 3'-to-5' exonucleolytic proofreading, the first few nucleotides laid down have a much higher rate of mismatch errors.
By utilizing a temporary RNA primer instead of starting directly with DNA, the cell ensures that these early, error-prone sequences are marked by their ribonucleotide backbone.
The subsequent removal and replacement of this RNA with high-fidelity DNA eliminates this potential source of genetic error.
Thus, Assertion (A) is true.
- Evaluating Reason (R): The RNA primers are eventually recognized and removed by the 5'-to-3' exonuclease activity of DNA Polymerase I (in prokaryotes) or FEN1/RNase H (in eukaryotes).
DNA polymerase then fills the resulting gaps with high-fidelity deoxyribonucleotides, allowing proofread base-pairing to be established across the entire replicated strand.
Thus, Reason (R) is true and directly explains why the use and subsequent removal of the RNA primer prevents permanent replication errors.

Step 2: Final Answer:
Both (A) and (R) are true, and (R) is the correct explanation of (A). This corresponds to option (A).