Tay-Sachs disease and cystic fibrosis are two examples of disorders caused by frameshift mutations. These mutations occur when a nucleotide is inserted or deleted, which changes how the DNA sequence is read.
During transcription, DNA is converted into messenger RNA. In the process of translation, ribosomes read mRNA sequences to produce the amino acids that become proteins. These mRNA sequences are read in units of three nucleotides called codons, and each codon corresponds to a single amino acid. The insertion or deletion of a nucleotide in a DNA sequence shifts the reading frame and changes how the rest of the sequence is read. As a result, different amino acids and proteins are produced. When mutations occur in multiples of three, however, another amino acid may be produced, but frameshift mutations do not occur because the reading frame is not altered.
Tay-Sachs disease occurs when the enzyme hexosaminidase A, which breaks down fatty acids in the brain, does not function properly. The resulting build up of acids in the brain causes deterioration of the nervous system. A frameshift mutation results in the proteins that comprise the enzyme not being formed correctly, causing Tay-Sachs disease. In the case of cystic fibrosis, the deletion of three nucleotides in the CFTR gene results in a frameshift mutation in which the alteration of the reading frame results in a stop codon and incorrect structure of the proteins produced by the gene.