Mutations are errors in codons caused by changes in nucleotide bases. Some mutations may not have much effect. For example, if the codon GAA becomes the codon GAG, because the genetic code is degenerate, the codon will still code for the amino acid glutamate. Such ineffectual mutations are called silent mutations. Some mutations, however, can have a huge affect on coding for amino acids, which can in turn affect what proteins are produced, which can have a profound effect on cellular and organismal function. The most common mutations occur in two ways: 1) a base substitution, in which one base is substituted for another; 2) an insertion or deletion, in which a base is either incorrectly inserted or deleted from a codon.
Base Substitutions Mutations
Base substitutions can have a variety of effects. The silent mutation cited above is an example of a base substitution, where the change in nucleotide base has no outward effect. A missense mutation refers to a base substitution when the change in nucleotide changes the amino acid coded for by the affected codon. A nonsense mutation refers to a base substitution in which the changed nucleotide transforms the codon into a stop codon. Such a change leads to a premature termination of translation, which can badly affect the formation of proteins.
When a nucleotide is wrongly inserted or deleted from a codon, the affects can be drastic. Called a frameshift mutation, an insertion or deletion can affect every codon in a particular genetic sequence by throwing the entire three by three codon structure out of whack. For example, given the code: GAU GAC UCC GCU AGG, which codes for the amino acids aspartate, aspartate, serine, alanine, arginine. If the A in the GAU were to be deleted, the code would become: GUG ACU CCG UAG G
There is a one other class of mutations, called suppressor mutations. These mutations are "mutations of mutations", which lead to a new type of change in the genetic code. There are two main classes of these mutations. A true reversion mutation occurs when there is a second mutation that restores the natural sequence of the genetic code. For example, a frameshift insertion could be suppressed by a frameshift deletion at a second position in the code. This type of supression is called intragenic suppression because it comes from within the genetic code.