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, because transitions are less likely to
result in amino acid substitutions, they are more likely to persist
and hence are more frequently observed in populations as single nucleotide
polymorphisms (SNPs) than
are transversions.
G)
or of one-ring pyrimidines
(C