What is the start codon in protein synthesis?

The start codon in protein synthesis is AUG, which is critical because it signals the beginning of translation—a key step in the process of gene expression. This codon not only indicates where the ribosome should begin assembling the amino acids into a protein, but it also codes for the amino acid methionine. Every new protein chain starts with methionine, and while it may be removed later, it serves as the first amino acid in the nascent polypeptide. The specific role of AUG in initiating translation highlights its importance in the overall mechanism of gene expression and the synthesis of proteins.

The other options do not serve as start codons; UAA is a stop codon, which signals termination of the protein synthesis process. GGC codes for the amino acid glycine, and ACG codes for threonine, neither of which is involved in the initiation of protein synthesis. Understanding the role of AUG as the start codon emphasizes the precise nature of genetic coding and the specific sequences required for the correct initiation of protein production.