“These pairs are: adenine (purine) with thymine (pyrimidine), and guanine (purine) with cytosine (pyrimidine).”

The last hurdle for Watson and Crick was to figure out how to arrange DNA’s four bases (adenine, thymine, guanine, and cytosine) inside the double helix without distorting the molecule. To visualize the answer, Watson built cardboard cutouts of the bases. Early one morning, as Watson moved the cutouts around on a tabletop, he found that the overall shape of an adenine molecule paired with a thymine molecule was similar to the overall shape of a guanine-cytosine pair. He immediately realized that arranging the bases in these pairs made a DNA structure without bulges or strains. Watson solved the puzzle "not by logic but serendipity," Crick recalled in his book What Mad Pursuit .

Watson and Crick picked up this model-building approach from eminent chemist Linus Pauling , who had successfully used it to discover that some proteins have a helical structure.