In 1944, Austrian biochemist, Erwin Chargaff read Oswald Avery's report that hereditary units were composed of DNA, and it encouraged him to begin his research on nucleotides. Eventually he discovered the composition of DNA. He discovered that:

- The number if adenine is equal to the number of thymine.
- The number of guanine is equal to the number of cytosine.
- The composition of DNA varies from one species to another.

This discovery helped James Watson and Francis Crick with their discoveries later on.

Erwin Chargaff