Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R
Assertion A: In the Watson-Crick model of DNA, each turn of the helix is the length of 10 base pairs.
Reason R: Each complete turn of the double-stranded DNA helix is 3.4 \r{A long.
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:

Remember the B-DNA numbers:
10 bp/turn.
3.4 \r{A} between bases.
34 \r{A} total turn length (Pitch).
20 \r{A} diameter.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question requires an understanding of the geometric dimensions of the B-form DNA double helix as proposed by Watson and Crick.

Step 2: Detailed Explanation:

• Assertion A Analysis: In the standard B-DNA model, there are 10 base pairs per complete \(360^{\circ}\) turn of the helix. This is a fundamental structural characteristic. Thus, Assertion A is True.

• Reason R Analysis:
- The distance between two consecutive base pairs (rise per base pair) is 3.4 \r{A.
- Since there are 10 base pairs in one full turn, the pitch (length of a complete turn) is:
\[ 10 \times 3.4 \text{ \r{A}} = 34 \text{ \r{A}} \]
- The statement claims a "complete turn" is 3.4 \r{A}, which is incorrect as that is the distance of only one step, not the full turn. Thus, Reason R is False.
Final Answer:
Assertion A is correct, but Reason R is incorrect. This corresponds to option (C).