Nonsense mutations
Nonsense mutations occur when a codon that is supposed to specify an amino acid is changed to a stop codon instead. This results in a protein that is shorter than normal because it cannot finish adding all the necessary amino acids. In DNA, the uracil (U) is replaced by thymine (T). If a change in the DNA sequence, or mutation, of a gene occurs that creates a stop codon, this is called a nonsense mutation. A nonstop mutation is a change in the sequence of a stop codon that turns it into a codon that specifies an amino acid.