A nonsense mutation is mutation under the classification of a point mutation. This means that it occurs through a single base change such as an A is changed to a G or a C is changed to a T. When a nonsense mutation occurs, a base is changed that results in a stop codon being transcribed into the mRNA before it is supposed to be. The early stop codon usually causes a nonfunctioning protein to be produced. The remaining amino acids beyond the changed base pair will not be transcribed due to the early stop codon being translated.
A nonsense suppressor can suppress a nonsense mutation. The suppressor is a mutated tRNA that recognizes the nonsense codon. Once recognized, when the ribosome comes to the nonsense codon, the tRNA may bind to it so that translation may continue. The tRNA suppresses all identical stop codons to the nonsense codon. This may not be such an issue due to tandem stop codons. Otherwise the protein would be extended. Another reason that the suppression of identical codons may not be a major problem is that tRNA repressors are not all that efficient. They may only repress every other or every third identical stop codon.