Transition versus Transversion mutations
DNA
substitution
mutations are of two types. Transitions are
interchanges
of two-ring purines (A 
G)
or
of one-ring pyrimidines
(C T):
they
therefore involve bases of similar shape.
Transversions
are interchanges of purine for
pyrimidine bases,
which therefore involve exchange of one-ring
and two-ring structures.
Although there are twice as many possible transversions, because of
the
molecular mechanisms by which
they are generated, transition mutations are generated at higher
frequency
than transversions. As well, transitions are less likely to
result in amino acid substitutions (due to "wobble"), and are therefore
more likely to persist as "silent substitutions" in populations as single nucleotide
polymorphisms (SNPs).
G)
or
of one-ring pyrimidines
(C