4. The number of triplet codon(s) for methionine is ____.

Number of Triplet Codons for Methionine: Understanding the AUG Codon in the Genetic Code

Introduction

The genetic code is the universal language through which genetic information stored in DNA is translated into proteins. During protein synthesis, every three consecutive nucleotides in messenger RNA (mRNA) form a codon, and each codon specifies either a particular amino acid or a termination signal. Since there are four different nucleotides, a total of 64 possible codons (4³) exist. Among these, 61 codons encode amino acids, whereas three codons function as stop signals.

One of the most important amino acids in protein synthesis is methionine. It is unique because it not only serves as an amino acid incorporated into proteins but also functions as the universal initiation signal for translation in almost all organisms. The codon that specifies methionine also serves as the start codon, marking the beginning of protein synthesis. This dual role makes methionine one of the most significant amino acids in molecular biology.

Correct Answer

Correct Answer: 1

Detailed Explanation

Methionine is encoded by only one codon in the standard genetic code:

AUG

The AUG codon has two important biological functions. First, it specifies the amino acid methionine during protein synthesis. Second, it acts as the initiation codon (start codon), signaling the ribosome to begin translation.

In eukaryotes, AUG recruits the initiator tRNA carrying methionine (Met-tRNA_i^Met). In bacteria, the same AUG codon recruits a specialized initiator tRNA carrying N-formylmethionine (fMet), which is a modified form of methionine used only for translation initiation.

Unlike many other amino acids that are encoded by multiple synonymous codons, methionine is specified by only a single codon. Therefore, the number of triplet codons encoding methionine is one.

Why Does Methionine Have Only One Codon?

The genetic code is described as degenerate because most amino acids are encoded by more than one codon. For example, leucine is encoded by six different codons, while glycine is encoded by four codons. Methionine and tryptophan are exceptions to this rule because each is encoded by only a single codon.

This lack of degeneracy means that every AUG codon specifies methionine, making translation highly accurate during initiation and protein synthesis.

Role of AUG in Translation Initiation

The AUG codon serves as the universal start signal for protein synthesis. During translation initiation, the small ribosomal subunit recognizes the AUG codon, allowing the initiator tRNA carrying methionine to bind. Once this complex is formed, the large ribosomal subunit joins the initiation complex, and protein synthesis proceeds by reading successive codons.

Thus, AUG performs two simultaneous functions:

• It specifies the amino acid methionine.
• It marks the beginning of translation.

Methionine in Prokaryotes and Eukaryotes

Codon Degeneracy for Selected Amino Acids

Properties of the Genetic Code

Biological Significance

The AUG codon plays a central role in gene expression because it determines the correct reading frame for translation. Accurate recognition of the start codon ensures that proteins are synthesized with the correct amino acid sequence. Errors in start codon recognition can lead to truncated proteins, altered reading frames, or complete failure of protein synthesis. The unique role of methionine as both an amino acid and the initiator of translation highlights its importance in molecular biology.

Final Answer

Methionine is encoded by only one triplet codon in the standard genetic code.

AUG → Methionine (Start Codon)

Number of triplet codons for methionine = 1

Correct Answer: 1