Figure 9.5: DNA Is a Double Helix (A) Francis Crick (right) and James Watson (left) proposed that the DNA molecule has a double-helical structure. (B) Biochemists can now pinpoint the position of every atom in a DNA molecule and have verified the essential features of the Watson–Crick model. Two sugar-phosphate chains provide the outer backbones of the double helix, with hydrogen-bonded nitrogenous bases forming “rungs” within the structure.