<P> He became an American citizen in 1940 . </P> <P> During his time at Columbia, Chargaff published numerous scientific papers, dealing primarily with the study of nucleic acids such as DNA using chromatographic techniques . He became interested in DNA in 1944 after Oswald Avery identified the molecule as the basis of heredity . In 1950, he discovered that the amounts of adenine and thymine in DNA were roughly the same, as were the amounts of cytosine and guanine . This later became known as the first of Chargaff's rules . </P> <P> Erwin Chargaff proposed two main rules in his lifetime which were appropriately named Chargaff's rules . The first and best known achievement was to show that in natural DNA the number of guanine units equals the number of cytosine units and the number of adenine units equals the number of thymine units . 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% . This strongly hinted towards the base pair makeup of the DNA, although Chargaff did not explicitly state this connection himself . For this research, Chargaff is credited with disproving the tetranucleotide hypothesis (Phoebus Levene's widely accepted hypothesis that DNA was composed of a large number of repeats of GACT). Most researchers had previously assumed that deviations from equimolar base ratios (G = A = C = T) were due to experimental error, but Chargaff documented that the variation was real, with (C + G) typically being slightly less abundant . He was able to do this with the newly developed paper chromatography and ultraviolet spectrophotometer . Chargaff met Francis Crick and James D. Watson at Cambridge in 1952, and, despite not getting along with them personally, he explained his findings to them . Chargaff's research would later help the Watson and Crick laboratory team to deduce the double helical structure of DNA . </P> <P> The second of Chargaff's rules is that the composition of DNA varies from one species to another, in particular in the relative amounts of A, G, T, and C bases . Such evidence of molecular diversity, which had been presumed absent from DNA, made DNA a more credible candidate for the genetic material than protein . </P>

Who is responsible for discovering the structure of dna
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