Question

Which of the following is not a feature of B-DNA ? A. Right handed double helix
B. 12 base pairs per turn of helix
C. Wide major grooves and narrow minor grooves
D. Anti glycosyl bond conformation at C and syn at G
E. Pitch per turn of helix \(33.2 \AA\)
Choose the correct answer from the options given below :

• A. \(A, C\) only
• B. \(B, E\) only
• C. \(C, D\) only
• D. \(B, D\) only

Hint:

Remember important features of B-DNA: \[ \text{Right-handed helix} \] \[ 10 \text{ bp per turn} \] \[ 34 \AA \text{ pitch} \]
• A-DNA \(\rightarrow\) About \(11-12\) bp/turn
• Z-DNA \(\rightarrow\) Left-handed helix with syn conformation at G

Answer 1

The Correct Option is D

Concept: B-DNA is the most common physiological form of DNA described by Watson and Crick. Important structural features of B-DNA include:
• Right-handed double helix
• Approximately \(10\) base pairs per turn
• Wide major groove and narrow minor groove
• Pitch of about \(34 \AA\)
• Bases generally in anti conformation

Step 1: Evaluating statement A. Statement A: \[ \text{Right handed double helix} \] This is a correct feature of B-DNA. Thus: \[ \boxed{A \text{ is correct}} \]

Step 2: Evaluating statement B. Statement B: \[ 12 \text{ base pairs per turn} \] This is incorrect because B-DNA contains approximately: \[ 10 \text{ base pairs per turn} \] while: \[ 12 \text{ bp per turn is characteristic of A-DNA} \] Thus: \[ \boxed{B \text{ is not a feature of B-DNA}} \]

Step 3: Evaluating statement C. Statement C: \[ \text{Wide major groove and narrow minor groove} \] This is a correct feature of B-DNA. Thus: \[ \boxed{C \text{ is correct}} \]

Step 4: Evaluating statement D. Statement D: \[ \text{Anti glycosyl bond conformation at C and syn at G} \] This arrangement is characteristic of: \[ \text{Z-DNA} \] In B-DNA: \[ \text{Both purines and pyrimidines are generally in anti conformation} \] Thus: \[ \boxed{D \text{ is not a feature of B-DNA}} \]

Step 5: Evaluating statement E. Statement E: \[ \text{Pitch per turn } = 33.2 \AA \] B-DNA pitch is approximately: \[ 34 \AA \] Hence this value is close enough to represent B-DNA characteristics. Thus: \[ \boxed{E \text{ is considered correct}} \] Therefore, the statements not representing B-DNA are: \[ \boxed{B \text{ and } D} \] Hence, the correct answer is: \[ \boxed{(D)\ B, D \text{ only}} \]