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: DNA can adopt several helical conformations depending on sequence, hydration and ionic conditions, and these are conventionally labelled A, B, C and Z form.

Step 2: A-form, B-form and C-form DNA are all right-handed double helices differing mainly in base pairs per turn (about 11, 10 and 9.3 respectively) and helical pitch.

Step 3: Z-DNA is distinctive because it winds in a left-handed, zig-zag helix, with about 12 base pairs per turn, a longer pitch than B-DNA, and it tends to form in alternating purine-pyrimidine sequences like GC repeats.

Step 4: Since the question specifies left-handed coiling with 12 base pairs per turn, matching the defining features of this unusual helix, the correct answer is Z form.