Beginning with its discovery in a gothic German castle, we follow the scientific surprises as DNA beat out protein, the more logical candidate, as the stuff of the gene. Joshua Lederberg pays tribute to the work of Oswald Avery, the Canadian-born scientist who first proved it was DNA and the impact this had on his own landmark discovery that bacteria have sex, i.e. exchange genetic material. Two sets of clues led to the discovery of the double helix structure of DNA, one from physics and the other from chemistry. James Watson recounts how he and Francis Crick put these clues together for the first time.