Final answer:
A series of three nucleotide bases signalling the end of a polypeptide chain is called a stop codon; it is one of the three nonsense codons that do not code for amino acids but instead terminate protein synthesis.
Step-by-step explanation:
A series of three nucleotide bases that signals the end of a polypeptide chain is a stop codon. A codon is a consecutive sequence of three nucleotides on an mRNA molecule that corresponds to a specific amino acid or a termination signal during the process of translation. While 61 of the 64 possible triplets code for amino acids, the remaining three are nonsense codons, or so-called stop codons: UAA, UAG, and UGA. These do not code for an amino acid and instead signal the termination of protein synthesis, releasing the polypeptide from the translation machinery.