Final answer:
The initiator codon codes for the amino acid methionine. It is represented by the codon AUG, which both initiates protein synthesis and codes for methionine, the first amino acid in a polypeptide chain.
Step-by-step explanation:
The initiator codon, also known as the start codon, codes for the amino acid methionine. In the genetic code, the codon AUG not only signifies the start of protein synthesis in both prokaryotes and eukaryotes, but it also codes for the incorporation of methionine as the first amino acid in a newly synthesized polypeptide chain. Although this initial methionine is typically removed before the polypeptide chain is completed, it is crucial during the initiation of translation. The AUG start codon is also unique in the sense that it is the only codon for methionine, as the genetic code is degenerate - meaning most amino acids are encoded by more than one codon.