Biology Reference
In-Depth Information
(pyrimidines), whereas
transversions
involve changes between a purine and a
pyrimidine.
An
insertion
or
deletion
(also called
indels
) involves the addition or deletion
of one or more nucleotides. An
inversion
is the excision of a part of the DNA
molecule followed by its reinsertion into the same position but with a reversed
orientation. An inversion in
Drosophila buzzatii
was caused by a transposable
element called
Galileo
, and this mechanism may be the mechanism by which
many inversions occur (
Caceres et al. 1999
).
Some mutations are lethal, whereas others have an effect on the organism
that can range from phenotypically undetectable (silent) to lethal only under
certain circumstances (
conditional lethal
). For example, many mutations are
temperature-sensitive, and the organism can survive if reared within one tem-
perature range but will die if reared at higher temperatures.
A
silent mutation
may occur if the third base in a codon is altered but,
because the genetic code is degenerate, there is no change in the amino acid
specified. These mutations are called
synonymous mutations
because there is no
change in protein structure or function from a silent mutation.
Some changes in codons alter the amino acid specified, and they are called
nonsynonymous
or
missense mutations
. Most point mutations that occur at the
first or second nucleotide positions of a codon will be missense, as will a few
third-position changes. A polypeptide with an amino-acid change may result
in a changed phenotype, depending on the precise role the altered amino acid
plays in the structure or function of the polypeptide. Most proteins can tolerate
some changes in their amino-acid sequence if the alteration does not change a
segment of the polypeptide essential for its structure or function.
Nonsense mutations
are point mutations that change a codon specifying an
amino acid into a termination codon, producing a truncated gene that codes
for a polypeptide that is terminated prematurely. In many cases, essential amino
acids will be deleted, and the protein's activity will be altered, resulting in a
mutant phenotype.
Frameshift mutations
result if additions or deletions of base pairs occur
that are not in a multiple of three. The polypeptide produced will likely have
a complete new set of amino acids produced downstream of the frameshift.
Frameshifts usually produce mutant phenotypes.
Occasionally,
back mutations
may occur to reverse a point mutation.
Reversions
sometimes occur when the original phenotype is restored by a new
