The observation that with DNA, as found within cells, for every A there is a T and for every G there is a C.
Since this observation was made prior to the invention of DNA sequencing, determination of the relative representation of adenine versus thymine versus guanine versus cytosine was achieved instead via chemical analysis.
Chargaff's rule was one of the clues that led to the 1953 proposal by Watson and Crick for the double helical nature of DNA.
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