- In the early 1950s, Rosalind Franklin and Maurice Wilkins used X-ray crystallography to determine the basic structure of DNA. In 1953, James Watson and Francis Crick used evidence from other scientists to determine that DNA has a double helix structure with nucleotides as monomers. - DNA is made up of four nitrogen bases (adenine, guanine, cytosine, thymine) that pair up in a specific way (A pairs with T and C pairs with G) to form the double helix structure. Millions of these base pairs are twisted together to form DNA.