Which one is 6-membered single-ring structure with N at 1st and 3rd position?

Options

A. Cytosine
B. Thymine
C. Adenine
D. Both (1) & (2)

Answer

To solve the question regarding the identification of a 6-membered single-ring structure with nitrogen (N) at the 1st and 3rd positions, we can follow these steps:

### Step 1: Understand the Structure
We need to identify a compound that has a six-membered ring structure. This means we are looking for a cyclic compound composed of six atoms, with at least two of those atoms being nitrogen.

### Step 2: Identify the Compounds
Let's consider the options typically associated with nucleobases: Cytosine, Thymine, and Adenine.

1. **Cytosine**: This compound has a single ring structure and contains nitrogen atoms. If we label the positions in the ring, nitrogen is present at the 1st and 3rd positions.

2. **Thymine**: Similar to Cytosine, Thymine also has a single ring structure and contains nitrogen atoms at the 1st and 3rd positions.

3. **Adenine**: This compound is different as it has a double-ring structure (purine) and does not fit the criteria of a six-membered single-ring structure.

### Step 3: Confirm the Nitrogen Positions
For both Cytosine and Thymine:
- In Cytosine, if we start counting from one nitrogen atom, the other nitrogen is indeed at the third position.
- In Thymine, the same counting applies, confirming nitrogen is at the 1st and 3rd positions.

### Step 4: Conclusion
Based on the analysis, both Cytosine and Thymine meet the criteria of being six-membered single-ring structures with nitrogen at the 1st and 3rd positions.

### Final Answer
The correct answer is both **Cytosine and Thymine**.

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