MEDICAL DICTIONARY
Mutation, nonsense: A mutation (a change) in a base in the DNA that prematurely stops the translation (reading) of messenger RNA ( mRNA ) resulting in a polypeptide chain that ends prematurely and a protein product that is truncated (abbreviated) and incomplete and usually nonfunctional.
The nonsense mutation converts a codon (a triplet of bases) that encodes an amino acid into a stop codon , one that specifies the termination of translation. There are three nonsense codons (UAG, UAA, and UGA) in mRNA. One of them comes normally at the end of each polypeptide. A nonsense codon is a stop codon which is out of place.
Catching such a defective messenger RNA (mRNA) before a truncated and potentially harmful protein is synthesized is the job of what is called the nonsense-mediated mRNA decay (NMD) pathway.
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