(A) : Guanine unites with Cytosine but not with Thymine.
(R ) : Guanine and Cytosine are purine bases while Thymine is a pyrimidine base.

Options

A. A and R are true and R explains A
B. A and R are true but R does not explain A
C. A is true and R is false
D. A is false and R is true

Answer

To analyze the given statements regarding the pairing of nucleobases, we will break down the assertion and reason step by step.

### Step 1: Understanding the Assertion
The assertion states: "Guanine unites with Cytosine but not with Thymine."

- **Explanation**: In DNA, guanine (G) pairs with cytosine (C) through three hydrogen bonds. This pairing is essential for the structure of DNA. However, guanine does not pair with thymine (T). Instead, thymine pairs with adenine (A).

### Step 2: Understanding the Reason
The reason states: "Guanine and Cytosine are purine bases while Thymine is a pyrimidine base."

- **Explanation**: This statement is partially correct. Guanine (G) is indeed a purine, and thymine (T) is a pyrimidine. However, cytosine (C) is also a pyrimidine, not a purine. The correct classification is:
- Purines: Adenine (A) and Guanine (G)
- Pyrimidines: Cytosine (C), Thymine (T), and Uracil (U)

### Step 3: Evaluating the Truth of Assertion and Reason
- The assertion is **true** because guanine does unite with cytosine but not with thymine.
- The reason is **false** because it incorrectly classifies cytosine as a purine.

### Conclusion
- **Assertion (A)**: True
- **Reason (R)**: False

Thus, the final answer is that the assertion is true, but the reason is false.

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