bonds for both the AT and the CG base pairs. As a result of this isomorphous geometry, the four base pair
combinations AT, TA, CG and GC can all be built into the same regular framework of the DNA duplex.
While Watson–Crick base pairing is the dominant pattern, other pairings have been suggested of which the
most significant to have been identified so far are Hoogsteen pairsand Crick ‘wobble’ pairs. Hoogsteen
pairs, illustrated for AT, are not isomorphous with Watson–Crick pairs because they have an 80° angle
between the glycosylic bonds and an 8.6 Å separation of the anomeric carbons (Figure 2.8). In the case of
reverse Hoogsteen pairs and reverse Watson–Crick pairs (not shown), one base is rotated through 180° rela-
tive to the other.
Francis Crick proposed the existence of ‘wobble’ base pairings to explain the degeneracy of the genetic
code (Section 7.3.1). This phenomenon calls for a single base in the 5-anticodon position of tRNA to be
able to recognise either of the pyrimidines or, alternatively, either of the purines as its 3-codon base partner.
Thus a GU ‘wobble’ pair has two hydrogen bonds, GN1HO2U and GO6HN3U, and
this requires a sideways shift of one base relative to its positions in the regular Watson–Crick geometry
(Figure 2.9). The resulting loss of a hydrogen bond leads to reduced stability which can be offset in part
by the improved base stacking(Section 2.3.1) that results from such sideways base displacement.
18 Chapter 2
Figure 2.7 Tautomeric equilibria for deoxycytidine showing hydrogen-bond acceptor aand donor dsites as used in
nucleic acid base pairing. The major tautomer for deoxyguanosine is drawn to show its characteristic
d.d.ahydrogen-bond donor–acceptor capacity
Figure 2.8 Watson–Crick base pairing for CG (left) and TA (centre). Hoogsteen base pairing for AT (right)
Figure 2.9 ‘Wobble’pairings for UG (left), UI (centre) and AI (right)