Which one of the following is not a pyrimidine base?

Options

A. Thymine
B. Guanine
C. Cytosine
D. Uracil

Explanation

To determine which of the following is not a pyrimidine base, we first need to understand the classification of nitrogenous bases in nucleic acids.

### Step-by-Step Solution:

1. **Identify the Types of Nitrogenous Bases**:
- Nitrogenous bases are categorized into two groups: **purines** and **pyrimidines**.
- Purines include adenine (A) and guanine (G).
- Pyrimidines include cytosine (C), thymine (T), and uracil (U).

2. **List the Given Options**:
- The options provided are:
1. Thymine (T)
2. Guanine (G)
3. Cytosine (C)
4. Uracil (U)

3. **Classify Each Option**:
- **Thymine**: This is a pyrimidine base.
- **Guanine**: This is a purine base.
- **Cytosine**: This is a pyrimidine base.
- **Uracil**: This is a pyrimidine base.

4. **Identify the Non-Pyrimidine Base**:
- From the classification, it is clear that **guanine (G)** is the only base listed that is not a pyrimidine. It belongs to the purine category.

5. **Conclusion**:
- Therefore, the answer to the question "Which one of the following is not a pyrimidine base?" is **guanine (G)**.

### Final Answer:
**Guanine (G)** is not a pyrimidine base.

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