What data was available to Watson and Crick?
Base composition of hydrolyzed samples of DNA Chargaff 1949-1953 separated four bases in DNA samples from various organisms then determined amounts of DNA from each source On the basis of his data, the following conclusions were drawn • The amount of A is proportional to the amount of T in DNA of any species . Also, the amount of G is proportional to the amount of C • Based on the above proportionality, the sum of purine (A+G) equals the sum of pyrimidines (T+C) • The percentage of C + G does not necessarily equal the percentage of A + T. As we can see the ratio between these two values varies greatly among species. Definite patterns of base compositions Xray diffraction studies of DNA Linus Pauling, Rosalind Franklin 3.4 angstrom periodicity (repeating unit) Franklin suggested structure was some sort of helix but did not propose definitive model Pauling proposed triple helix The Watson-Crick model Built model under constraints discussed Their model has the following features Two long
