Information Technology Reference
In-Depth Information
It is important to analyze more closely the effects of point mutation on the
protein itself. In a gene, the replacement of one nucleotide by another can
have different effects: (1) the new codon might code for a new amino acid
(these are called missense mutations); (2) the new codon might code for a
“stop” codon, truncating the protein, or else a “stop” codon mutates into an
amino acid codon, elongating the chain (nonsense mutations); (3) fairly fre-
quently, though, point mutations in a gene have no effect at all in the protein
sequence as the new codon might code for the same amino acid (neutral
mutations); (4) in addition, in eukaryotes, where the sequence of most genes
is interrupted by noncoding regions (introns), if a mutation occurs in an intron
it has no effect whatsoever in protein sequence (also an example of a neutral
mutation).
In another kind of protein mutation, large or small fragments may be in-
serted or deleted in the coding region of a gene. Large insertions or deletions
almost invariably result in the production of a defective protein. The effect
of short insertions or deletions depends on whether or not these modifica-
tions cause a shift in the reading frame of the gene (if they do, they are called
frameshift mutations). If the fragment deleted or inserted is a multiple of
three, then one or more codons are removed or added, resulting in the dele-
tion or insertion of one or more amino acids in the protein. The consequences
of these non-frameshift deletions/insertions are similar to the ones caused by
missense mutations.
The effects of these kinds of mutations on the structure and functionality
of a protein can be quite different. Point mutations may be neutral in effect,
either not changing the amino acid at all or changing it by another that func-
tions equally well in that position. The deletion/insertion of codons may also
be of little consequence, changing only slightly the protein function. Occa-
sionally, such mutations increase the efficiency of a protein, conferring some
selective advantage for the organism itself. On the other hand, nonsense
mutations and frameshift mutations have, almost every time, a lethal effect,
especially if the new protein is fundamental to the survival of the organism.
Nonetheless, very occasionally, such mutations might give rise to new, revo-
lutionary traits.
We will see that, in gene expression programming, most mutations, in-
cluding point mutations and small insertions, have a profound effect in the
structure and function of expression trees, more resembling the nonsense
and frameshift mutations that occur in nature. Nonetheless, this type of
