Complementary base pairing
Watson and Crick came up with the principle of complementary base pairing when they suggested a structure for DNA. The basic principles are that a purine base always pairs with a pyrimidine base. In particular, adenine will base pair with thymine and form two hydrogen bonds and cytosine will base pair with guanine and form three hydrogen bonds. Base pairing in this way is vital for explaining the mechanisms of DNA replication, transcription, translation and DNA repair.