A third approach by James Watson and Francis Crick of the Cavendish Laboratory in Cambridge which ultimately reveled the DNA structure involved building a model of the DNA molecule that incorporated information they could gather.
Two lines of evidence proved crucial .Firstly, both Watson and Crick had access to the X-ray diffraction data of Wilkin and Franklin. Secondly , they realised the significance of the evidence published by Erwin Chagaff in 1951 concerning the ratio of the different nucleotide bases known to be present in DNA
From the X-ray diffraction photographs, Watson and crick deduced that there were two helices present in DNA .from Chargaff’s data, they realized that the bases of DNA were always present in fixed pairs . that is, adenine paired with thymine, and guanine paired with cytosine.