Final answer:
A group of 3 nucleotides that specify either an amino acid or a translation stop signal is called a codon.
Step-by-step explanation:
A group of 3 nucleotides that specify either an amino acid or a translation stop signal is called a codon. Codons are consecutive sequences of three nucleotides on an mRNA molecule that correspond to specific amino acids. They determine the sequence and order of amino acids in a polypeptide chain during translation.