Final answer:
Methionine is the amino acid that appears at the N-terminus of all newly translated prokaryotic and eukaryotic polypeptides, even though it is often removed during post-translational processing.
Step-by-step explanation:
The amino acid that appears at the N-terminus of all newly translated prokaryotic and eukaryotic polypeptides is methionine. In prokaryotes, specifically organisms such as E. coli, this methionine is modified to N-formylmethionine (fMet), which is inserted into the growing polypeptide chain by a special tRNA known as initiator tRNA. In eukaryotes, the methionine is not modified, but still serves as the initial amino acid during the translation process. Although methionine is the first amino acid, it is often removed during post-translational processing, resulting in mature proteins that may not have methionine at their N-terminus. The codon AUG, which codes for methionine, serves as the start codon that initiates protein synthesis.