How many nucleotides are found in one spiral of B-DNA?

Options

A. 5
B. 10
C. 20
D. 25

Answer

To determine how many nucleotides are found in one spiral of B-DNA, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Structure of B-DNA**:
B-DNA is a right-handed double helix structure. It has a specific diameter and a characteristic number of base pairs per turn.

2. **Identify the Key Characteristics**:
- The diameter of B-DNA is approximately 2 nanometers.
- Each complete turn of the helix contains about 10.5 base pairs.

3. **Calculate the Number of Nucleotides**:
Since each base pair consists of two nucleotides (one from each strand of the DNA), we can calculate the total number of nucleotides in one complete turn of B-DNA.
- Number of base pairs per turn = 10.5
- Therefore, the number of nucleotides in one turn = 10.5 base pairs × 2 nucleotides/base pair = 21 nucleotides.

4. **Determine the Answer**:
The question asks for the number of nucleotides in one spiral or turn of B-DNA. Based on our calculation, there are approximately 21 nucleotides in one complete turn of B-DNA.

5. **Choose the Closest Option**:
Among the options provided (5, 10, 20, 25), the closest option to our calculated value of 21 nucleotides is option 3, which is 20.

### Final Answer:
The number of nucleotides found in one spiral of B-DNA is approximately **20**.

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