What specifies the first amino acid in protein synthesis?

The start codon is critical in protein synthesis because it indicates the site at which translation begins. This codon, typically AUG, not only marks the starting point for the ribosome to assemble proteins but also encodes for the first amino acid, methionine, in the nascent polypeptide chain. Once the ribosome encounters this start codon on the mRNA, it recruits the appropriate tRNA molecule, which carries the methionine, and initiates the translation process. This foundational role of the start codon underscores the importance of correctly identifying where the protein synthesis will begin, ultimately determining the sequence and structure of the resulting protein.

In contrast, other choices do not play a direct role in specifying the initial amino acid. For instance, stop codons signal the termination of protein synthesis, while anticodons are involved in pairing with mRNA codons to ensure the correct amino acids are added during elongation. Additionally, the coding sequence refers to the portion of DNA or mRNA that dictates the specific amino acid sequence of the protein but does not specifically indicate where translation initiates.