Which of the following is only a ribose nucleotide?

Options

A. Cytosine-pentose sugar-phosphate
B. Guanine-pentose sugar-phosphate
C. Thymine-pentose sugar-phosphate
D. Uracil-pentose sugar-phosphate

Answer

To determine which of the given options is only a ribose nucleotide, we need to understand the structure of a nucleotide and the types of sugars involved.

### Step-by-Step Solution:

1. **Understand what a nucleotide is**:
A nucleotide consists of three components: a nitrogenous base, a pentose sugar, and a phosphate group. The pentose sugar can either be ribose or deoxyribose.

2. **Identify the types of sugars**:
- **Ribose**: A five-carbon sugar found in RNA.
- **Deoxyribose**: A five-carbon sugar found in DNA.

3. **Analyze the options**:
- **Option A: Cytosine, Pentose Sugar, and Phosphate**:
- Cytosine can be part of both RNA (with ribose) and DNA (with deoxyribose). Therefore, it is not exclusively a ribose nucleotide.
- **Option B: Guanine, Pentose Sugar, and Phosphate**:
- Similar to cytosine, guanine is found in both RNA (with ribose) and DNA (with deoxyribose). Hence, this option is also not exclusively ribose.
- **Option C: Thymine, Pentose Sugar, and Phosphate**:
- Thymine is only found in DNA, which contains deoxyribose sugar. Thus, this option cannot be a ribose nucleotide.
- **Option D: Uracil, Pentose Sugar, and Phosphate**:
- Uracil is found only in RNA, which contains ribose sugar. Therefore, this option is exclusively a ribose nucleotide.

4. **Conclusion**:
Based on the analysis, the only option that represents a ribose nucleotide is **Option D: Uracil, Pentose Sugar, and Phosphate**.

### Final Answer:
**Option D: Uracil, Pentose Sugar, and Phosphate** is the only ribose nucleotide.

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