Final answer:
A base pair substitution that introduces a premature stop codon and potentially truncates the polypeptide chain is termed a nonsense mutation.
Step-by-step explanation:
A base pair substitution that results in the introduction of a premature stop codon and potentially truncates the polypeptide chain is known as a nonsense mutation. These mutations replace an amino acid codon with a stop codon, which leads to the premature termination of protein synthesis. An example of a nonsense mutation would be changing the DNA sequence from AGCGTA (coding for Serine and Valine) to AGCGTA (coding for Serine and a Stop), which truncates the protein.