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 B-DNA Structure**:
- B-DNA is a double helical structure composed of two strands that wind around each other. Each strand is made up of nucleotides.

2. **Identify the Components**:
- Each nucleotide consists of a sugar, a phosphate group, and a nitrogenous base. The four types of nitrogenous bases in DNA are adenine (A), thymine (T), guanine (G), and cytosine (C).

3. **Determine the Number of Base Pairs in One Turn**:
- In B-DNA, one complete turn of the helix contains approximately 10 base pairs. A base pair consists of two nucleotides, one from each strand.

4. **Calculate the Total Number of Nucleotides**:
- Since each base pair consists of two nucleotides, if there are 10 base pairs in one turn, the total number of nucleotides can be calculated as:
\[ \text{Total Nucleotides} = \text{Number of Base Pairs} \times 2 \]
\[ \text{Total Nucleotides} = 10 \times 2 = 20 \]

5. **Conclusion**:
- Therefore, in one spiral of B-DNA, there are 20 nucleotides.

### Final Answer:
One spiral of B-DNA contains **20 nucleotides**.

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