One of these is not an amino acid:

Options

A. Tryptophan
B. Uracil
C. Lysine
D. Glycine

Answer

To determine which of the given options is not an amino acid, we need to understand the basic structure of an amino acid. Amino acids are organic compounds characterized by the presence of both an amino group (-NH2) and a carboxylic acid group (-COOH), along with a variable side chain known as the R group.

### Step-by-Step Solution:

1. **Understand the Structure of Amino Acids**:
- Amino acids consist of three main components:
- An amino group (-NH2)
- A carboxylic acid group (-COOH)
- A side chain (R group) that varies among different amino acids.

2. **Analyze Each Option**:
- **Option A: Tryptophan**:
- Tryptophan has an amino group, a carboxylic acid group, and a side chain (indole group).
- Therefore, Tryptophan is an amino acid.

- **Option B: Uracil**:
- Uracil has an amino group but lacks a carboxylic acid group.
- It does not fit the basic structure of an amino acid, making it not an amino acid.

- **Option C: Lysine**:
- Lysine contains an amino group, a carboxylic acid group, and a side chain (which is a long hydrocarbon chain with an amino group).
- Thus, Lysine is an amino acid.

- **Option D: Glycine**:
- Glycine has an amino group, a carboxylic acid group, and its side chain is simply a hydrogen atom (making it the simplest amino acid).
- Therefore, Glycine is also an amino acid.

3. **Conclusion**:
- After analyzing all the options, we find that Uracil is the only compound that does not meet the criteria of an amino acid.
- Therefore, the correct answer is **Option B: Uracil**.