Diameter and pitch per turn of helix of 'A' model of DNA is

Options

A. 26Å & 25Å
B. 20Å & 34Å
C. 18Å & 46Å
D. 19Å & 20Å

Answer

To solve the question regarding the diameter and pitch per turn of the helix of the 'A' model of DNA, we will follow these steps:

### Step-by-Step Solution:

1. **Understand the Structure of DNA**:
- DNA (Deoxyribonucleic Acid) is a molecule that carries genetic information. It has different structural forms, including A-DNA, B-DNA, and Z-DNA. Each form has distinct physical properties.

2. **Identify the A-DNA Characteristics**:
- A-DNA is one of the forms of DNA that is right-handed and is typically found under dehydrated conditions. It is more compact than B-DNA.

3. **Know the Measurements for A-DNA**:
- The diameter of A-DNA is approximately 25.5 Å (Angstroms), which can be rounded to 26 Å for simplicity.
- The pitch of A-DNA, which is the distance for one complete turn of the helix, is about 25 Å.

4. **Analyze the Options Given**:
- The question provides four options for the diameter and pitch:
- a) 26 Å and 25 Å
- b) 20 Å and 34 Å
- c) 18 Å and 46 Å
- d) 19 Å and 20 Å

5. **Select the Correct Answer**:
- Based on the known measurements of A-DNA, the correct answer is option (a) 26 Å for diameter and 25 Å for pitch.

6. **Conclusion**:
- Therefore, the diameter and pitch per turn of the helix of the A model of DNA is 26 Å and 25 Å, respectively.

### Final Answer:
**The diameter and pitch per turn of the helix of the A model of DNA is 26 Å and 25 Å.**

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