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stop codon Any of the 3 codons (or triplets of nucleotides—UAA, UAG and UGA) on an mRNA molecule which lack a cognate aminoacyl tRNA—i.e., do not code for an amino acid and thus signal the end of protein synthesis. When ribosomes hit a stop codon, mRNA transcription is terminated, causing polypeptide synthesis to stop and the ribosome to dissociate from the mRNA.
nonsense codon One of the three nucleotide triplets (codons), UAG, UAA or UGA that mark an end point to a particular protein synthesis. U is the base uracil; A is the base adenine; and G is the base guanine.