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.
Insertion/Deletion Mutations
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
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.
Suppressor Mutations
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.
