DNA is Double Helix

DNA is shown in the double-stranded helical model for the graphic on the left. The easiest way to visualize DNA is an immensely long rope ladder, twisted into a cork-screw shape. The sides of the ladder are alternating sequences of deoxyribose and phosphate (backbone) while the rungs of the ladder (bases) are made in two parts with each part firmly attached to the side of the ladder. The parts in the rung are heterocyclic amines held in position by hydrogen bonding. Although most DNA exists as open ended double helices, some bacterial DNA has been found a cyclic helix. Occasionally, DNA has been found as a single strand.