Which of the following is not a pyrimidine base

Options

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

Answer

To determine which of the following options is not a pyrimidine base, we need to identify the nature of each base provided in the options. The options given are uracil, thymine, cytosine, and guanine.

### Step-by-Step Solution:

1. **Identify the Bases**:
- The bases provided are:
- Uracil
- Thymine
- Cytosine
- Guanine

2. **Classify Each Base**:
- **Uracil**: This is a pyrimidine base that is found in RNA.
- **Thymine**: This is also a pyrimidine base, found in DNA.
- **Cytosine**: This is another pyrimidine base, present in both DNA and RNA.
- **Guanine**: This is a purine base, which is different from pyrimidines. It is found in both DNA and RNA.

3. **Conclusion**:
- Among the options provided, uracil, thymine, and cytosine are all pyrimidine bases, while guanine is a purine base. Therefore, the base that is not a pyrimidine is **guanine**.

### Final Answer:
Guanine is not a pyrimidine base.

---