Question

The first amino acid in any polypeptide chain of eukaryotes is

• A. Leucine
• B. Methionine
• C. Iso-Leucine
• D. Lysine

Hint:

The universal start codon is AUG, which always codes for Methionine (\(\text{Met}\)) as the first amino acid in eukaryotic protein synthesis.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Translation is the process of decoding mRNA transcripts to synthesize proteins.
Translation initiation requires a specific start codon and a dedicated initiator tRNA to place the first amino acid of the polypeptide chain.

Step 2: Detailed Explanation:
In both prokaryotes and eukaryotes, the standard genetic code uses the codon 5'-AUG-3' as the primary translation start signal.
This codon specifies the insertion of the amino acid methionine.
During eukaryotic translation initiation, a specialized initiator tRNA (\(\text{tRNA}_i^{\text{Met}}\)) loaded with methionine binds to the start codon in the ribosomal P-site, making methionine the first amino acid incorporated into almost all nascent polypeptide chains.
In prokaryotes, the initiator tRNA carries a chemically modified form of methionine called N-formylmethionine (fMet).
In eukaryotes, the initiator tRNA carries unmodified methionine.
Although this initial methionine is often cleaved off post-translationally by methionine aminopeptidase, it is always the first amino acid incorporated during polypeptide synthesis.
Leucine (A), isoleucine (C), and lysine (D) do not serve as initiator amino acids.

Step 3: Final Answer:
The first amino acid is Methionine, corresponding to option (B).