Number of phosphodiester bonds in one turn of DNA

Options

A. 16
B. 20
C. 18
D. 9

Answer

To determine the number of phosphodiester bonds in one turn of DNA, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Phosphodiester Bonds**:
- Phosphodiester bonds are the connections formed between the sugar molecule and the phosphate group in the DNA backbone.

2. **Identifying Base Pairs in One Turn of DNA**:
- In one complete turn of DNA, there are approximately 10 base pairs.

3. **Determining Phosphodiester Bonds Between Base Pairs**:
- Each adjacent pair of base pairs is connected by phosphodiester bonds. For 10 base pairs, we need to find out how many connections (or bonds) there are.

4. **Calculating the Number of Phosphodiester Bonds**:
- Since there are 10 base pairs, the number of adjacent pairs is one less than the number of base pairs. Therefore, the number of adjacent pairs is 10 - 1 = 9.
- Each adjacent pair is connected by 2 phosphodiester bonds (one for each strand of the DNA).
- Thus, the total number of phosphodiester bonds is calculated as:
\[ \text{Total Phosphodiester Bonds} = \text{Number of Adjacent Pairs} \times 2 = 9 \times 2 = 18 \]

5. **Conclusion**:
- Therefore, the total number of phosphodiester bonds in one turn of DNA is **18**.

### Final Answer:
- The correct option is **C: 18**.