DNA strands contain coded information in the form of nucleotide permutations for the synthesis of proteins. Nucleotides are composed of a five-carbon sugar, a phosphate group and a nitrogenous base. The four types of bases are complementarily paired together between the two strands, where adenine pairs with thymine with two hydrogen bonds whilst cytosine and guanine pair with three hydrogen bonds. Three nucleotides form a codon, and this corresponds to the production of a specific amino acid – this process is repeated to produce a chain of polypeptides or proteins. Mutations are changes to the base sequence of the nucleotides which ultimately affect protein synthesis – this can lead to phenotypic disorders and conditions. If a changes in base sequence occurs it can have no effect on the overall organism, or it can be classified under four types of mutations: substitution, insertion, deletion and frameshift.
Substitution mutations occur when one base is swapped with another, e.g. an adenine is replaced by guanine, this can change the amino acids in proteins. There are three types of substitution mutations: silent, missense and nonsense. A silent mutation is when a change in a base has no impact and the same amino acid is produced from the codon – this occurs because there are many codons that code for the same amino acid. Missense mutation is when a substitution occurs and produces a different amino acid, which can be beneficial or detrimental to overall protein function. Nonsense mutations are when a base is substituted with another to create a stop codon, this produces an incomplete protein.
Insertion mutations are when extra base pairs are added to the sequence, this can have the effect of adding stop codons to produce incomplete proteins. Deletion mutations are when portions of the base sequence are deleted which then certain genes are not expressed leading to genetic disorders. Frameshift mutation is when a combination of insertions and deletions of the base sequence occur, the sequence is no longer divisible by three and it changes the codons, amino acids and proteins expressed.