Initiation of translation — Which amino acid is used universally to start protein synthesis at the AUG start codon (noting formylation differences in bacteria)?

Introduction:
Translation begins at a start codon, typically AUG, which codes for methionine. Bacteria use a specialized initiator tRNA charged with N-formylmethionine (fMet), whereas eukaryotes and archaea use methionine without formylation. Understanding this unity and variation is central to molecular genetics.

Given Data / Assumptions:

• Standard genetic code assigns AUG to methionine.
• Initiator tRNAs are distinct from elongator tRNAs.
• Formylation occurs in bacteria and mitochondria but not in cytosolic eukaryotic translation.

Concept / Approach:
Map codon to amino acid and compare across domains of life. While some alternative starts occur, the biochemical identity inserted is methionine (formylated in bacteria), making methionine the universal initiating amino acid under the canonical code.

Step-by-Step Solution:

Verification / Alternative check:
Protein N-termini often begin with Met in eukaryotes; in bacteria, initiator fMet may be deformylated and the initial Met removed post-translationally, consistent with methionine as the starting residue.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing alternative start codons in some bacteria with different amino acids; those codons still recruit the initiator tRNA for methionine/fMet.

Final Answer:
Methionine (fMet in bacteria, Met in eukaryotes/archaea).