Erwin Chargaff's most important discovery for understanding DNA was his rule that base pairs always come in the same percentages. Scientists knew that DNA was likely made up of four base pairs (adenine, guanine, cytosine and thymine), but they thought they were in equal percentages. A base pair is the amino acid chain that is the building block of a DNA molecule. Chargaff showed that adenine and thymine had equal percentages, and cytosine and guanine had their own set ratio.
As a follow up to his first rule (also sometimes referred to as a regulation), Chargaff showed that these base-pair percentages changed depending on which organism the DNA came from (though, no matter the organism, adenine and thymine were equal, and cytosine and guanine were paired). Scientists previously had believed that DNA was basically the same in all organisms, and thus was not involved in creating complexity. By showing that different organisms have different types of DNA, Chargaff paved the way for the consideration that DNA was much more complex than previously considered.
Chargaff worked on his rules in the mid-to-late 1940s, and he met with Watson and Crick in 1952. Chargaff at the time did not realize the significance of his rules (he understood base pairs and that they hinted at complexity, but he did not understand their true significance), and he was a harsh critic of Watson and Crick, especially after they won the Nobel Prize and he did not. Chargaff prided himself on being an outspoken critic of what he saw as bad and sloppy work in biology.
Erwin Chargaff was born on August 11, 1905, in what was then the Austrian-Hungarian Empire (now modern-day Ukraine). He got his Ph.D. in chemistry from the University of Vienna in 1928, and he then subsequently studied at Yale, the University of Berlin and Columbia, where he stayed from 1935 until 1974; Chargaff left Columbia on unpleasant terms. He was married and had one son. Erwin Chargaff died on on June 20, 2002.