Across the centre of the DNA double helix:
Adenine (A) always pairs up with Thymine (T);
Guanine (G) always pairs up with Cytosine (C).
This purine-pyrimidine pairing ensures that the rungs of the DNA ladder are the same length.
Although these four images are rotatable in 3 dimensions, it is very noticeable that each molecule is in fact flat! Click here
to turn all 4 through 90° to show them from the edge - 3 more clicks to return to the start position!
In each case, an asterisk (*) marks the position where the base attaches to C1 of deoxyribose by condensation (H atom removed).
Numbers represent atoms between which hydrogen bonds form across the centre of the DNA molecule (2 such hydrogen bonds in the case of A-T and 3 in the case of C-G).