Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): DNA replication is very accurate.
Reason (R): DNA polymerase add correct base with correct hydrogen bonding of the base pair and the common geometry of the standard \(\text{A}=\text{T}\) and \(\text{G}\equiv\text{C}\) base pairs recognized by the active site of polymerase enzyme.
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:

DNA replication achieves its low error rate through three sequential steps:
1. Active site geometric selection (errors: 1 in \(10^5\)).
2. 3'\(\rightarrow\)5' Exonuclease proofreading (errors reduced to 1 in \(10^7\)).
3. Post-replication mismatch repair (final errors: 1 in \(10^9\) to \(10^{10}\)).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Fidelity in DNA replication is essential to prevent mutations and preserve genetic information across generations. This high accuracy is maintained through active selection by DNA polymerase during synthesis.

Step 2: Detailed Explanation:
- Assertion Analysis: DNA replication has an error rate of only 1 mistake per \(10^9\) to \(10^{10}\) base pairs added, making it an exceptionally accurate biological process, validating the Assertion.
- Reason Analysis: The active site of DNA polymerase has strict geometric constraints. Correct Watson-Crick base pairs (\(\text{A}=\text{T}\) and \(\text{G}\equiv\text{C}\)) have identical structural dimensions and fit neatly into the catalytic pocket of the enzyme. Incorrectly paired bases do not fit this geometry, preventing the polymerase from forming the phosphodiester bond. This active geometric selection, along with proofreading, ensures high fidelity, validating the Reason.
- Since the geometric selection of correct Watson-Crick base pairs by the DNA polymerase active site explains why replication is so accurate, (R) is the correct explanation of (A).

Step 3: Final Answer:
Both (A) and (R) are true, and (R) is the correct explanation of (A).