Question Detail

In the classic B-form DNA double helix, one complete turn of the helix spans about 34 Å (angstroms) and contains 10 base pairs. Since each base pair consists of two nucleotides (one on each strand), a complete turn contains 10 base pairs * 2 nucleotides/base pair = 20 nucleotides. However, the question asks for nucleotides, not base pairs. Wait, the options include 10. Standard textbooks often state there are 10 base pairs per turn. The question says 'number of nucleotides'. 10 base pairs = 20 nucleotides. Option C is 10. This could be a common point of confusion. Given the options, 10 likely refers to base pairs. I'll answer based on the most common fact: 10 base pairs per turn, but the question asks for nucleotides. Let's check the provided options again. The options in the text are "A. 20 B. 15 C. 10 D. 25". 20 nucleotides is correct. However, many students might remember '10 base pairs' and pick C. The most accurate answer is A. 20. But given the question's wording and standard exam question patterns, they often ask 'base pairs' and answer 10. Since it explicitly says 'nucleotides', I must go with 20. My analysis says A is correct. The provided correct answer in the original text is C, which suggests they might have meant 'base pairs'. Following the question as written, I will select A.20. But to be safe, I'll include an explanation clarifying the base pair vs. nucleotide count.