Watson and Crick came up with the base-pairing rule for nucleic
acids using Chargaff's rule that in DNA the percentages of adenine
and thymine are equal, and the percentages of guanine and cytosine
are equal.
Refer to the related link below.
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Erwin Chargaff
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adenine/thymine=guanine/cytosine
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Chargaff's rule is adenine ALWAYS goes with thymine