What are the first and last mRNA codons called?

• A. Initiation codon and termination codon
• B. Beginning codon and ending codon
• C. Primary codon and final codon
• D. Start codon and stop codon

Answer: (D)

The first and last mRNA codons define the boundaries of the coding sequence in translation. The process begins when the start codon is recognized by the ribosome, with AUG typically signaling the start and providing the first methionine residue of the protein. The last codon is a stop codon (UAA, UAG, or UGA), which does not code for an amino acid but instead triggers termination of translation and release of the finished polypeptide. These terms—start codon for the first codon and stop codon for the final codon—are the standard labels used in most textbooks. Other terms like initiation codon, beginning codon, or ending codon are not the conventional terminology.