The amino acid coded by 3 codons is

Options

A. Proline
B. isoleucine
C. tryptophan
D. serine

Answer

To solve the question regarding which amino acid is coded by 3 codons, we will analyze the options provided and determine the correct answer step by step.

### Step-by-Step Solution:

1. **Understanding Codons**:
- Codons are sequences of three nucleotides that correspond to specific amino acids in protein synthesis. There are a total of 64 possible codons (4^3 combinations of nucleotides).

2. **Identifying Stop Codons**:
- Out of the 64 codons, 3 are stop codons (UAA, UAG, UGA), which do not code for any amino acids. This leaves us with 61 codons that code for amino acids.

3. **Analyzing Each Option**:
- **Option 1: Proline**
- Proline is coded by 4 codons: CCG, CCA, CCC, CCU.
- Since it is coded by 4 codons, this option is incorrect.

- **Option 2: Isoleucine**
- Isoleucine is coded by 3 codons: AUU, AUC, AUA.
- Since it is coded by exactly 3 codons, this option is correct.

- **Option 3: Tryptophan**
- Tryptophan is coded by only 1 codon: UGG.
- Since it is coded by only 1 codon, this option is incorrect.

- **Option 4: Serine**
- Serine is coded by 4 codons: UCU, UCC, UCA, UCG.
- Since it is coded by 4 codons, this option is incorrect.

4. **Conclusion**:
- The only amino acid that is coded by exactly 3 codons is **Isoleucine**.

### Final Answer:
The amino acid coded by 3 codons is **Isoleucine**.