Final answer:
The first amino acid incorporated at the N-terminus of a nascent polypeptide chain is methionine, which is coded by the start codon AUG, and it signifies the start of the protein translation process.
Step-by-step explanation:
The first amino acid incorporated at the N-terminus of a nascent polypeptide chain is methionine, which is coded by the start codon AUG. This is true for both prokaryotic and eukaryotic organisms. The N-terminal of a protein is the end where the amino group of the first amino acid is free, and in the representation of proteins, the N-terminal amino acid is written first, on the left side of the diagram.
Once the polypeptide chain has been synthesized, this initial methionine may be enzymatically removed before the polypeptide is completed. In the majority of functional proteins, methionine is not the first amino acid. However, the presence of methionine at the beginning of the translation process is crucial as it signifies the starting point of protein synthesis.