Editing Erwin Chargaff (section)

==Chargaff's rules==

Key conclusions from Erwin Chargaff's work are now known as [[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]]<ref>{{cite journal | title = Yeast nucleic acid | last= Levine | first = P.A.  | journal = Biochem. Z.  | date = 1909 | volume= 17  | pages= 120–131 
}}</ref> ([[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 did his experiments 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,<ref>{{cite book |title=Landmark Experiments in Molecular Biology|author=Michael Fry|date=2016|publisher=Academic Press|page=220|isbn=978-0-12-802074-6}}</ref> 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.

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]].<ref name="baianu10">{{cite book |last1=Baianu |first1=I.C. |title=Mathematical and Theoretical Biology. Volume 2 – Mathematical and Molecular Biophysics |date=2 February 2010 |publisher=I.C. Baianu |url=https://archive.org/details/MathematicalAndMolecularBiophysics-quantumPhysicsAndTechniques/AMathMolBiophys_Vol2p328/mode/2up}}</ref>