<span>the race to determine how these pieces fit together in a three-dimensional structure was won by James Watson and Francis Crick at the Cavendish Laboratory in Cambridge, England. They showed that alternating deoxyribose and phosphate molecules form the twisted uprights of the DNA ladder. The rungs of the ladder are formed by complementary pairs of nitrogen bases — A always paired with T and G always paired with C so they form a ladder like shape</span>
A DNA molecule is like a ladder because there are two sides (the sugar-phosphate backbones of the two strands) and rungs (each rung being a pair of complementary bases). Twisted, because the molecule is twined in a coil called a helix - or rather, a double helix because there are two strands.
