How many bases are encompassed in one complete 360-degree turn of the DNA helix?

How many bases are encompassed in one complete 360-degree turn of the DNA helix?

• A. 8
• B. 12
• C. 10
• D. 9

Answer: (C)

In B-form DNA, one full turn of the helix corresponds to about 3.4 nanometers of length, and each base pair stacks 0.34 nanometers apart. Divide the turn length by the rise per base pair: 3.4 nm / 0.34 nm per base pair = 10 base pairs per turn. So there are 10 base pairs, which means 20 individual bases total when counting both strands, per full 360-degree turn. The standard measure used in questions like this is base pairs per turn, so 10 is the correct value.

In B-form DNA, one full turn of the helix corresponds to about 3.4 nanometers of length, and each base pair stacks 0.34 nanometers apart. Divide the turn length by the rise per base pair: 3.4 nm / 0.34 nm per base pair = 10 base pairs per turn. So there are 10 base pairs, which means 20 individual bases total when counting both strands, per full 360-degree turn. The standard measure used in questions like this is base pairs per turn, so 10 is the correct value.