Watson and Crick used the central rule to arrange DNA molecules. They first put forward the triple helix structure with bases on the outside, then the double helix structure with bases on the inside, and finally put forward the double helix structure with complementary bases, and built a model according to this idea, and finally successfully discovered the double helix structure of DNA molecules. As early as 1930s, the chemical composition of DNA was determined. It consists of four basic units of nucleotides, each of which consists of a base, a deoxyribose and a phosphate group. There are four bases, namely adenine, guanine, thymine and cytosine. It is these previous discoveries that enabled Watson and Crick to successfully discover the double helix structure of DNA molecules.