Base pairing is one of the key feature of the DNA double helix, helping to explain how genetic information is inherited when cells divide. While nucleobases can form hydrogen bonds in many different arrangements, the overall structure of DNA in a double helix provides strong constraints, giving rise to the two canonical base pairs shown in the initial scene (). Historically, it was difficult to provide structural evidence of base pairing without solving the structure of a DNA helix fragments of ten or more base pairs. The first crystal structure of a base pair was solved by Hoogsteen in 1963 and was not canonical (it is called a Hoogsteen pair now). A theoretical analysis of possible base pairs was difficult because the lowest energy tautomers of bases were unknown or disputed, making it impossible to distinguish possible hydrogen bond donors and acceptors.
