Question

The number of base pairs present in each turn of B-DNA helix is:

• A. 9
• B. 10
• C. 11
• D. 12

Hint:

Keep the key parameters of DNA forms clear: B-DNA has 10 bp/turn, A-DNA (dehydrated form) has 11 bp/turn, and Z-DNA (left-handed helix) has 12 bp/turn.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Double-stranded DNA can adopt several distinct structural conformations depending on hydration levels, salt concentrations, and base sequence.
The most common physiological conformation in living cells is B-DNA, which was originally described by Watson and Crick.

Step 2: Detailed Explanation:
B-DNA is characterized by several specific helical parameters:
It is a right-handed double helix.
The distance between adjacent base pairs along the helical axis (rise per base pair) is approximately \( 4\text{ \AA} \) (0.34 nm).
The helical pitch, which is the physical length of one complete turn of the helix (\( 360^\circ \) rotation), is approximately \( 34\text{ \AA} \) (4 nm).
The average number of base pairs per turn is calculated as: \[ \text{Base pairs per turn} = \frac{\text{Helical Pitch}}{\text{Rise per Base Pair}} = \frac{34\text{ \AA}}{4\text{ \AA}} = 10 \text{ base pairs} \]
In aqueous solution, the exact value is approximately 4 to 5 base pairs per turn, but classically and in standard textbook definitions, it is represented as 10 base pairs per turn.

Step 3: Final Answer:
There are 10 base pairs present in each complete turn of the B-DNA helix.