Question

The DNA that shows left-handed coiling, contains 12 base pairs per turn and has a pitch of 69 Å is known as __________ DNA.

• A. A form
• B. B form
• C. C form
• D. Z form

Hint:

Only one of these four DNA forms winds in a left-handed direction.

Answer 1

The Correct Option is D

Step 1: Among the various DNA helix forms described in structural biology, A, B and C forms are all right-handed helices, differing from each other mainly in how tightly the base pairs are stacked and how many base pairs occur per full turn.

Step 2: Z-DNA stands apart as the only common left-handed double helix, with its backbone tracing a zig-zag path rather than a smooth curve, giving it roughly 12 base pairs per turn and a longer helical pitch than the right-handed forms.

Step 3: Because the question explicitly describes left-handed coiling with 12 base pairs per turn, both hallmark features of this zig-zag helix, the correct identification is Z form DNA.