Final answer:
Chargaff's findings contradicted Levene's tetranucleotide hypothesis by revealing varying nucleotide ratios among species, with A equalling T and G equalling C within each species, which were key to forming the DNA double helix model.
Step-by-step explanation:
If Levene's tetranucleotide hypothesis were correct, Erwin Chargaff would have found the nucleotide ratios of adenine (A), thymine (T), guanine (G), and cytosine (C) to be equal, meaning that the ratio of A:T and G:C would be 1:1. However, Chargaff's actual findings disproved this hypothesis. He found that the concentrations of these nucleotide bases varied among species, but within a species, the amount of A always equaled the amount of T, and the amount of G always equaled the amount of C, contradicting the tetranucleotide hypothesis. These findings, known as Chargaff's rules, were critical in the development of the DNA double helix model by Watson and Crick.