Induction of transition
mutations by spontaneous tautomeric
In the original
in the standard (amino) form pairs with T.
replication, the two strands separate. In the upper diagram, T
pairs with A as usual, which
the wild-type sequence. In the lower diagram, A
has undergone a tautomeric shift to the non-standard (imino)
A', which pairs with C.
In the next round of replication, the imino A'
shifts back to the amino A form,
pairs with T, which again
the wild-type sequence. Replication of the other strand pairs C
with G. By comparison with the original molecule, the
A tautomeric shift in one strand has produced a
mutation in the complementary strand.
Note that a tautomeric shift is not
mutation, but a transient change to an alternative form of the
on this diagram, demonstrate that the tautomeric shifts of the
three bases also produce transition mutations.