Assertion :- Constant diameter of DNA double helix is `20Å`
Reason : Purines always pair with pyrimidines and vice-versa

Options

A. If both Assertion & Reason are True & the Reason is a correct explanation of the Assertion.
B. If both Assertion & Reason are trur but Reason is not a correct explanation of the Assertion.
C. If Assertion is True but the Reason is Flase.
D. If both Assertion & Reason are false.

Answer

To solve the question, we need to evaluate the assertion and the reason provided:

**Assertion:** The constant diameter of DNA double helix is 20 Å.
**Reason:** Purines always pair with pyrimidines and vice-versa.

### Step-by-Step Solution:

1. **Evaluate the Assertion:**
- The assertion states that the diameter of the DNA double helix is 20 Å. This is a well-established fact in molecular biology. The DNA double helix maintains a consistent diameter due to its structural properties.
- **Conclusion:** The assertion is **true**.

2. **Evaluate the Reason:**
- The reason states that purines always pair with pyrimidines and vice-versa. In DNA, the purines (adenine and guanine) pair specifically with the pyrimidines (thymine and cytosine). Adenine pairs with thymine (A-T) through two hydrogen bonds, while guanine pairs with cytosine (G-C) through three hydrogen bonds.
- This complementary base pairing is crucial for maintaining the structure of the DNA double helix and is indeed a fundamental principle of DNA structure.
- **Conclusion:** The reason is also **true**.

3. **Determine the Relationship Between Assertion and Reason:**
- The reason explains why the assertion is true. The constant diameter of the DNA double helix is maintained by the specific pairing of purines with pyrimidines. If two purines or two pyrimidines were to pair, it would disrupt the consistent diameter of the helix.
- Therefore, the reason is a correct explanation of the assertion.

4. **Final Conclusion:**
- Both the assertion and the reason are true, and the reason correctly explains the assertion.

### Final Answer:
Both assertion and reason are true, and reason is the correct explanation of the assertion.

---