Assertion :- Base pairing between puring and pyrimidines allows uniform distance between two strands of the helix.
Reason : Number of hydrogen bonds between pairing bases are constant.

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 analyze the assertion and the reason provided.

### Step 1: Understand the Assertion
The assertion states that "Base pairing between purine and pyrimidines allows uniform distance between two strands of the helix."

- In DNA, the bases are classified into two categories: purines (adenine and guanine) and pyrimidines (cytosine and thymine).
- Base pairing occurs between a purine and a pyrimidine: adenine pairs with thymine (A-T) and cytosine pairs with guanine (C-G).
- The uniform distance between the two strands of the DNA helix is maintained because of this specific pairing (A-T and C-G), which allows the DNA structure to remain stable and consistent.

**Conclusion for Step 1**: The assertion is true.

### Step 2: Understand the Reason
The reason states that "Number of hydrogen bonds between pairing bases are constant."

- Adenine (A) and thymine (T) are connected by 2 hydrogen bonds.
- Cytosine (C) and guanine (G) are connected by 3 hydrogen bonds.
- Since the number of hydrogen bonds differs between the A-T pair and the C-G pair, it is not constant.

**Conclusion for Step 2**: The reason is false.

### Step 3: Final Evaluation
Now that we have evaluated both the assertion and the reason:

- The assertion is true.
- The reason is false.

Thus, the correct answer to the question is that the assertion is true, and the reason is false.

### Final Answer:
Assertion is true, and Reason is false.

---