The amino acid coded by 3 codons is

Options

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

Answer

To determine which amino acid is coded by exactly 3 codons, we can analyze the options provided based on the number of codons that correspond to each amino acid.

### Step-by-Step Solution:

1. **Understand Codons and Amino Acids**:
- There are a total of 64 codons in the genetic code.
- Out of these, 61 codons code for amino acids, while 3 are stop codons that do not code for any amino acid.

2. **Identify the Amino Acids and Their Codons**:
- **Proline**:
- Codons: CCA, CCG, CCU, CCC (Total: 4 codons)
- **Isoleucine**:
- Codons: AUA, AUU, AUC (Total: 3 codons)
- **Tryptophan**:
- Codon: UGG (Total: 1 codon)
- **Serine**:
- Codons: UCU, UCC, UCA, UCG (Total: 4 codons)

3. **Count the Codons for Each Amino Acid**:
- Proline: 4 codons
- Isoleucine: 3 codons
- Tryptophan: 1 codon
- Serine: 4 codons

4. **Identify the Amino Acid Coded by 3 Codons**:
- From the analysis, we see that only **Isoleucine** is coded by exactly 3 codons (AUA, AUU, AUC).

5. **Conclusion**:
- The amino acid coded by 3 codons is **Isoleucine**.

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

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