Watson and Crick model of DNA is

Options

A. B-form DNA with a spiral length of 34 Å and a diameter of 20 Å
B. A-form DNA with a spiral length of 15 Å and a diameter of 20 Å
C. Z-form DNA with a spiral length of 34 Å and a diameter of 20 Å
D. B-form DNA with a spiral length of 28Å and a diameter of 14Å

Answer

### Step-by-Step Solution:

1. **Understanding the Watson and Crick Model**:
- The Watson and Crick model of DNA was proposed in 1953. It describes the double-helix structure of DNA.

2. **Key Features of the Model**:
- The DNA is in the B form.
- The diameter of the DNA helix is approximately 20 Ångströms (Å).
- The helical turn (spiral length) of the DNA is about 34 Ångströms.
- Each complete turn of the helix contains about 10 base pairs.

3. **Analyzing the Options**:
- **Option 1**: States the B form of DNA with a diameter of 20 Å and a spiral length of 34 Å. This is correct according to the Watson and Crick model.
- **Option 2**: Describes an A form of DNA with a spiral length of 15 Å and a diameter of 20 Å. This is incorrect as it does not match the B form characteristics.
- **Option 3**: Refers to Z form DNA, which is not applicable in this context as the Watson and Crick model specifically describes the B form.
- **Option 4**: Mentions a P form DNA with a spiral length of 28 Å, which is also incorrect since the spiral length in the B form is 34 Å.

4. **Conclusion**:
- The correct statement regarding the Watson and Crick model of DNA is the first option, which accurately describes the B form of DNA.