What are Chargaff's Rules?
His lab abandoned all other research and focused solely on DNA. At the time the tetranucleotide hypothesis was doing the rounds. It said that DNA was made up of four bases (adenine, thymine, guanine and cytosine) that were present in regular amounts, but that their organisation was too simple to carry hereditary information. It was Chargaff's work that overturned this.
By looking at the composition of DNA from various organisms i.e. plants, people and fish, Chargaff noticed that in any given species the ratio of adenine to thymine was roughly equal, and the ratio of cytosine and guanine was also roughly equal. For example in human DNA the ratios are; A=30.9%, T=29.4%, G=19.9% and C=19.8%. This rule is the same for every living thing. He had managed to show that the chemical bases exhibited a complementary relationship.
Even though Chargaff made this discovery and published the research in 1950, the significance of these base-pairs (he did not use that phrase) was to elude him. It wasn't until Crick and Watson's paper on the structure of DNA three years later that it was fully understood. In fact they cited his work in their landmark paper.
One of his other major observations was that the composition of DNA varied from species to species, in particular the ratios of the chemical bases.
Both these observations are now known as Chargaff's Rules, and it is likely that without them Crick and Watson's work would not have been possible. In fact both did pay tribute to the debt they owed to Chargaff's research.