A+T and G+C base pairs have similar structures
The combination of a two-ring purine (A or G) and a single-ring pyrimidine
(T or C) forms a base bair (A+T or G+C ) with three
co-planar rings. The distance between the sugar components of the
backbones of either strand is essentially identical for
either pair.
The A+T has two H-bonds versus the G+C with three. It may be convenient to think
of an "A" as having two toes that are
paired to the two "ears" of the "T".
Figure © 2012 TA Brown, Introduction to Genetics (1st
ed.); additional text © 2014 by Steven M. Carr