What mRNA codon always serves as the start codon?

What mRNA codon always serves as the start codon?

• A. UAG
• B. AUG
• C. UAA
• D. UGA

Answer: (B)

Start translation at the codon that the ribosome recognizes as the initiation signal. AUG is the start codon because it is the one recognized by the initiator tRNA, which carries methionine in eukaryotes (and formyl-methionine in bacteria), and it establishes the reading frame for the whole protein. This codon effectively marks where the ribosome begins assembling the amino acid chain and where translation starts. The other codons shown are stop signals (UAG, UAA, UGA) that tell the ribosome to terminate, not to begin making protein, so they cannot serve as the starting point. Rare exceptions exist in some organisms where alternative start codons are used, but AUG is by far the standard start codon in most contexts.

Start translation at the codon that the ribosome recognizes as the initiation signal. AUG is the start codon because it is the one recognized by the initiator tRNA, which carries methionine in eukaryotes (and formyl-methionine in bacteria), and it establishes the reading frame for the whole protein. This codon effectively marks where the ribosome begins assembling the amino acid chain and where translation starts. The other codons shown are stop signals (UAG, UAA, UGA) that tell the ribosome to terminate, not to begin making protein, so they cannot serve as the starting point. Rare exceptions exist in some organisms where alternative start codons are used, but AUG is by far the standard start codon in most contexts.