a, The double-helix structure of DNA is held together by
specific interactions (dotted lines) between pairs of bases: adenine (A) pairs
with thymine (T), and guanine (G) pairs with cytosine (C). b, The
DNA bases form rare isomeric structures known as tautomers, which can allow the
formation of mispairs; bonds shown in blue are the tautomeric forms of the bonds
shown in red in a. Kimsey et al.1 have detected tautomeric
G·T mispairs in DNA duplexes, and conclude from modelling studies that
this explains the frequency with which G·T is misincorporated into DNA
during DNA duplication by polymerase enzymes — as proposed3 by Watson and
Crick in 1953.