Question

One turn of the helix in a B-form DNA is approximately

• A. 3.4 nm
• B. 2 nm
• C. 20 nm
• D. 0.34 nm

Answer 1

Answer: (A)

3.4 nm

• In a DNA molecule, the two strands are not parallel but intertwined with each other. Each strand looks like a helix.
• The two strands form a "double helix" structure, which was first discovered by James D. Watson and Francis Crick in 1953.
• In this structure, also known as the B form, the helix makes a turn every 3.4 nm, and the distance between two neighbouring base pairs is 0.34 nm.

So, the correct answer is '3.4 nm'.