A missense mutation is one which results in the replacement of one amino acid in a polypeptide chain by another. As a result of mutation one base of a codon may be substituted by another base.
The changed codon may then code for another amino acid. A missense mutation can be caused by substitution, deletion or insertion.
Missense mutations arising by substitution result in proteins which differ from their normal counterparts only in a single amino acid. Such proteins therefore frequently have normal biological activity.
One
of
the
codons
for
phenylalanine
is
UUU.
A
single
base
substitution
(U-->G)
changes
it
to
UGU,
the
codon
for
cysteine
Thus
the
protein
formed
after
mutation
is
identical
to
the
normal
protein
except
that
phenylalanine
is
substituted
by
cysteine.
About
half
the
known
human
haemoglobins
have
amino
acid
substitutions
involving
single
base
transversions.