Assertion: Adenine cannot pair with cytosine.
Reason: Adenine and cytosine do not have a perfect match between hydrogen donar and hydrogen accepter.

Options

A. If both Assertion and Reason are true and the Reason is the correct explanation of the Assertion.
B. If both Assertion and Reason are true and the Reason is not the correct explanation of the Assertion.
C. If Assertion is true but the Reason is false.
D. If both Assertion and Reasom are false.

Answer

**Step-by-Step Solution:**

1. **Understanding the Assertion:**
- The assertion states that "Adenine cannot pair with cytosine."
- In the context of DNA and RNA, adenine (A) is a purine, while cytosine (C) is a pyrimidine.

2. **Understanding Base Pairing:**
- Nucleotides in DNA and RNA pair according to specific rules.
- Purines (adenine and guanine) pair with pyrimidines (cytosine, thymine, and uracil) to maintain a consistent width in the DNA double helix.

3. **Identifying Correct Pairings:**
- According to Chargaff's rules, adenine pairs with thymine (A-T) in DNA, and guanine pairs with cytosine (G-C).
- Therefore, adenine cannot pair with cytosine, confirming the assertion is true.

4. **Understanding the Reason:**
- The reason states that "Adenine and cytosine do not have a perfect match between hydrogen donor and hydrogen acceptor."
- Base pairing involves hydrogen bonds between the nitrogenous bases, where specific positions on the bases act as hydrogen donors and acceptors.

5. **Analyzing the Hydrogen Bonding:**
- Adenine forms two hydrogen bonds with thymine, while guanine forms three hydrogen bonds with cytosine.
- The lack of a proper hydrogen bonding pattern between adenine and cytosine further supports that they cannot pair effectively.

6. **Conclusion:**
- Both the assertion and the reason are true. The assertion is correct because adenine does not pair with cytosine, and the reason is also correct as it explains the basis of why this pairing does not occur.

**Final Answer:**
Both the assertion and the reason are true, and the reason correctly explains the assertion.

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