English: Double helical structures of DNA or even RNA are known to be stabilized by Watson-Crick base pairing between the purines, Ade and Gua, with the pyrimidines, Thy (or Ura for RNA) and Cyt respectively. In this scheme, the N1 atoms of purine residues form hydrogen bond with N3 atoms of pyrimidine residues in A:T and G:C complementarity. The second hydrogen bond in A:T base pairs involves the N6 atom of Ade and the O4 atom of Thy. Similarly, the second hydrogen bond in G:C base pairs involves O6 and N4 atoms of Gua and Cyt respectively. The G:C base pairs also have a third hydrogen bond involving the N2 atom of Gua and the O2 atom of Cyt. However, though the same Watson-Crick hydrogen bonding pattern was observed between guanine and cytosine when co crystallized in vitro, a different hydrogen bonding pattern was observed between adenine and thymine when similarly co crystallized. Thus, the first high resolution structure of a Ade:Thy base pair, as solved by Hoogsteen, showed that there are two hydrogen bonds involving N7 and N6 atoms of Ade and N3 and O4 (or O2) atoms of Thy, respectively