Edwin Chargaff (August 11, 1905 - June 20, 2002) was an Austrian chemist. Chargaff was born in Czernowitz, Bukowina, Austria. From 1923-1928. He studied chemistry in Vienna, then two years in Yale. From 1930 on he worked at the University of Berlin, until he switched to the Pasteur Institute in Paris in 1933. In 1935 he emigrated to New York. Chargaff became a professor at Columbia University.
Of Chargaff's four rules on DNA base composition, only his first parity rule was incorporated into mainstream biology as the DNA double helix . Now, the cluster rule, the second parity rule, and the GC rule, reveal the multiple levels of information in our genomes and potential conflicts between them.
The first parity rule states that double-stranded DNA from any cell of all organisms have a 1:1 ratio of pyrimidine and purine bases and more specifically that the amount of guanine is equal to cytosine and the amount of adenine is equal to thymine. In human DNA, for example, the four bases are present in these percentages: A=30.9% and T=29.4%; G=19.9% and C=19.8%. Although Chargaff did not offer any explanation for his results and refused to speculate, it was clear that they were not in accordance with Watson's like-with-like model which he regarded as being 'unsound'. This discovery helped Watson and Crick in their discovery of the double helical structure of DNA .
The cluster rule showed that base clustering in microorganisms often relates to transcription direction. The "top" strand of part of a DNA duplex which is transcribed contains pyrimidine clusters if transcription is to the left of the promoter, and purine clusters if transcription is to the right of the promoter.
Where Chargaff's first parity rule suggested base pairing between two strands in a pair, his second parity rule suggested intrastrand base pairing in what are now known as stem-loop secondary structures .
The fourth rule, the GC rule, was demonstrated before the genetic code was deciphered. This states that the amino acid composition of the proteins of microorganisms is influenced, not just by the demands of the environment on the proteins, but also by the base composition of the genome encoding those proteins.