Erwin Chargaff

Chargaff

AT base pair

GC base pair

In 1950 he experimentally determined certain facts about the composition of DNA that led directly to the correct elucidation of its molecular structure. In particular, he demonstrated three rules (now known as Chargaff's Rules), which state that in DNA:

• the number of adenine residues always equals the number of thymine residues;
• the number of guanine residues always equals the number of cytosine residues;
• the number of purines (A+G) always equals the number of pyrimidines (T+C).

Chargaff's findings strongly suggested that base-pairing existed within DNA between adenine and thymine, and between guanine and cytosine (see figures below right), and that other possible pairings such as (A-C, G-T, A-A, T-T, C-C, or G-G) do not occur. These are the basic facts you have to know to construct an accurate model of the DNA double helix.

Two years later he explained these findings to James Watson and Francis Crick, who were then able quickly to elucidate the double-helix structure of DNA. As Chargaff himself later put it,

"I told them all I knew. If they had heard before about the pairing rules, they concealed it. But as they did not seem to know much about anything, I was not unduly surprised. I mentioned our early attempts to explain the complementarity relationships by the assumption that, in the nucleic acid chain, adenylic was always next to thymidylic acid and cytidylic next to guanylic acid....I believe that the double-stranded model of DNA came about as a consequence of our conversation."*

During the 1950s, Chargaff took controversial and outspoken stances that probably antagonized many of his colleagues. For example, he claimed that the "technology of genetic engineering poses a greater threat to the world than the advent of nuclear technology." Such comments might have contributed to his not being included in the Nobel Prize for discovery of the structure of DNA. Instead, Francis Crick, James Watson and Maurice Wilkins were recognized.