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 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
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
In other words, every single codon would code for a new amino acid, resulting in completely different proteins coded for during translation. The physical results of such mutations are, understandably, usually catastrophic.
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.
The other class of supressor mutation is called extragenic supression because the second mutation does not occur in the gene. As we shall see in Translation, transfer RNA (tRNA), plays an important mediating role in the translation of mRNA information into actual proteins. For example, if codon UAC, which normally codes for the amino acid tyrosine, mutates into UAG, a stop codon, the result is a nonsense mutation. But if the tRNA that is specifically designed to "fetch" tyrosine also mutates, so that it now binds with the former stop codon mRNA sequence, then the effect of the two mutations negate each other. This last type of mutation will make a lot more sense after we have more thoroughly discussed tRNA and its function in translation.