Erwin Chargaff was the one who analysed the base content of DNA and says the ratio of the cell.There are 2 Chargaff's Rule: ⇒ Variability - the amount of A,T,G,C in DNA varies from species to species. ⇒ Constancy - The percentage of A=T and the amount of G=C (equal)
Rosalind Franklin was the one who discover the model of the DNA molecule. And said that DNA is a helix.
James Watson and Francis Crick were the ones who studied the DNA molecule Franklin discovered and they are the ones who designed the DNA double helix which we are using today.