Question

Which of the following is/are true for B-DNA?
(A). It is Right-handed
(B). It has 10.4 base pairs per turn of helix
(C). It's helix diameter is broadest among A and Z type
(D). The glycosidic bond is of anti type

Choose the correct answer from the options given below:

• A. (A), (B) and (C) only.
• B. (A), (C) and (D) only.
• C. (B), (C) and (D) only.
• D. (A), (B) and (D) only.

Hint:

Remember the diagnostic features of the three DNA forms:
A-DNA: Broadest, right-handed, 11 bp/turn, anti glycosidic bonds.
B-DNA: Intermediate, right-handed, 10.4 bp/turn, anti glycosidic bonds.
Z-DNA: Narrowest, left-handed, 12 bp/turn, alternating syn/anti glycosidic bonds.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Double-stranded DNA can adopt multiple helical conformations depending on the sequence, hydration level, and salt concentrations.
The three major conformations found in biological systems are A-DNA, B-DNA, and Z-DNA. B-DNA is the standard conformation adopted by genomic DNA under physiological conditions.

Step 2: Detailed Explanation:
Let us evaluate each structural property of B-DNA:
- Statement (A) is true: B-DNA is a right-handed double helix.
- Statement (B) is true: Under physiological aqueous conditions, B-DNA has a helical pitch containing approximately 10.4 base pairs per complete helical turn.
- Statement (C) is false: The helical diameter of B-DNA is approximately \( 2.0\text{ nm} \).
This is narrower than A-DNA, which has a diameter of \( 2.3\text{ nm} \).
This makes A-DNA the broadest helix, while Z-DNA is the narrowest with a diameter of \( 1.8\text{ nm} \).
- Statement (D) is true: The conformation of the glycosidic bond (connecting the deoxyribose sugar to the nitrogenous base) in B-DNA is exclusively in the anti conformation for all nucleotides.
In contrast, Z-DNA contains alternating purines and pyrimidines, where the purines adopt the syn conformation and pyrimidines adopt the anti conformation.
Therefore, statements (A), (B), and (D) are correct, while statement (C) is incorrect.

Step 3: Final Answer:
The correct option is (D) (A), (B) and (D) only.