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Mutations are changes in DNA structure or amount. Nonsense mutations are single base substitutions that change a codon to a stop codon, resulting in a shorter mRNA strand and truncated protein. Nonsense mutations cause genetic diseases like cystic fibrosis, Duchenne muscular dystrophy, beta thalassemia, and Hurler syndrome.
Whenever there is a change in the structure or amount of DNA in an organism, it is referred to as a mutation. There are different types of mutations, depending on where the change occurs and how the DNA is affected. A nonsense mutation is a single base substitution or a point mutation. When a nonsense mutation occurs, only one base in the DNA strand is changed.
DNA consists of a double helix of long strands of nucleotides to which nitrogenous bases are attached. The four bases are adenine, thymine, guanine and cytosine. Nucleotides join together to form long strands of polynucleotides. The two strands are held together by hydrogen bonds between complementary base pairs.
Each base is either a purine base or a pyrimidine base depending on the number of chemical rings in its molecular structure. Purine bases have two chemical rings and include adenine and guanine. Pyrmidine bases have only one chemical ring and include cytosine and thymine. Purine bases always pair with pyrimidine bases, or more specifically, adenine with thymine and cytosine with guanine.
When proteins are synthesized in a cell, the information for each protein is carried along the DNA strands in the genes. A base triplet, or codon, in a gene codes for a specific amino acid, which is a building block of protein molecules. The sequence of amino acids to be used to form a protein is determined by the sequence of the codons. Any changes in these codons, if not repaired, can affect the protein being formed.
To form a protein, two processes occur. First, DNA is transcribed into messenger RNA (mRNA) inside the nucleus. mRNA is a single-stranded molecule, so it’s able to leave the nucleus through tiny holes, or pores, in the nuclear membrane. The mRNA now carries the information to synthesize a protein during translation in the cytoplasm of the cell.
Nonsense mutations affect the genetic coding region of DNA. When a nonsense mutation occurs, a base is changed so that the triplet code for its amino acid changes to code for a stop codon. Stop codons are TTA, TAG or TGA. A nonsense mutation causes the transcription of DNA to mRNA to stop, meaning that the mRNA strand is shorter than it should be. The earlier this mutation occurs in the gene, the shorter the mRNA will be and the more truncated the protein will be, making it less likely to work.
There are a number of genetic diseases caused by nonsense mutations. Cystic fibrosis is caused by mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) protein. Some patients have a nonsense mutation in the gene, which prevents much of the protein from being formed. Any defects in this protein cause the different symptoms of the condition. Other conditions that can be caused by nonsense mutations include Duchenne muscular dystrophy, beta thalassemia, and Hurler syndrome.
