Nonsense mutations

In genetics, a nonsense mutation is a point mutation in a sequence of DNA that results in a premature stop codon, or a nonsense codon in the transcribed mRNA, and in a truncated, incomplete, and usually nonfunctional protein product. It differs from a missense mutation, which is a point mutation where a single nucleotide is changed to cause substitution of a different amino acid. Some genetic disorders, such as thalassemia and DMD, result from nonsense mutations.

Suppose that a nonsense mutation was introduced at the fourth triplet in the DNA sequence (CGA) causing the cytosine to be replaced with thymine, yielding TGA in the DNA sequence. Since TGA is transcribed-then-translated as UGA, the resulting transcript and protein product would be:

The remaining codons of the mRNA are not translated into amino proteins because the stop codon is prematurely reached during translation. This can yield a truncated abbreviated protein product, which quite often lacks the functionality of the normal, non-mutant protein.

Despite an expected tendency for premature termination codons to yield shortened polypeptide products, in fact the formation of truncated proteins does not occur often in vivo. Many organisms—including humans and lower species, such as yeast—employ a nonsense-mediated mRNA decay pathway, which degrades mRNAs containing nonsense mutations before they are translated into nonfunctional polypeptides.

The three stop codons have been given names: UAG is amber, UGA is opal (sometimes also called umber), and UAA is ochre. "Amber" was named by discoverers Richard Epstein and Charles Steinberg after their friend Harris Bernstein, whose last name means "amber" in German. The other two stop codons were named "ochre" and "opal" in order to keep the "color names" theme. Stop codons are also called "termination" or "nonsense" codons. They signal release of the nascent polypeptide from the ribosome because there is no cognate tRNA that has anticodons complementary to these stop signals, and so a release factor binds to the ribosome instead.

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