Erwin Chargaff (11 August 1905 – 20 June 2002) was an Austro-Hungarian biochemist who immigrated to the United States during the Nazi era and was a professor of biochemistry at Columbia University medical school. Through careful experimentation, Chargaff discovered two rules that helped lead to the discovery of the double helix structure of DNA.
The first rule was that in DNA the number of guanine units is equal to the number of cytosine units, and the number of adenine units is equal to the number of thymine units. This hinted at the base pair makeup of DNA.
The second rule was that the relative amounts of guanine, cytosine, adenine and thymine bases vary from one species to another. This hinted that DNA rather than protein could be the genetic material.