Which statement about start codons and initiating amino acids is correct?

• A. AUG; methionine; bacteria use formyl-methionine (fMet) to initiate.
• B. AUG; methionine; both bacteria and eukaryotes use methionine without formyl.
• C. UGG; tryptophan; bacteria initiate.
• D. GUG; valine; bacteria initiate.

Answer: (A)

Start codons set where translation begins, and the usual initiating amino acid is methionine. The difference between bacteria and eukaryotes shows up in the exact initiating form: in bacteria, the initiator methionine is formylated to formyl-methionine (fMet) before the ribosome starts the chain, while in eukaryotes the initiator carries plain methionine. So AUG as the start codon, methionine as the initiating amino acid, and the bacterial use of formyl-methionine to start all align in this statement. The other options misstate either the start codon, the initiating amino acid, or the bacterial initiation specifics. Some bacterial genes can start with alternative codons like GUG or UUG, but initiation still uses formyl-methionine, not valine or tryptophan, so those choices don’t fit.