Erwin Chargaff, 1949
Chargaff rules
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Chargaff's rules state that in the DNA of every species and every organism, the amount of guanine should be equal to the amount of cytosine and the amount of adenine should be equal to the amount of thymine. In addition, there should be a stoichiometric ratio of purine and pyrimidine bases of 1:1 (i.e. A+G=T+C). This pattern is found in both strands of DNA. This principle was discovered by the Austrian-born chemist Erwin Chargaff in the late 1940s.
First rule of parity
The first rule states that a double-stranded DNA molecule has an overall percentage of base pair identity: A% = T% and G% = C%. The rigorous confirmation of this rule forms the basis of the Watson-Crick base pairs in the DNA double helix model.
Second parity rule
The second rule states that both Α% ≈ Τ% and G% ≈ C% are valid for each of the two DNA strands (Rudner R, Karkas JD, Chargaff E (1968). This only describes a global property of the base composition in a single DNA strand. It states that in single-stranded DNA, the number of adenine units is approximately equal to that of thymine (%A ≈ %T) and the number of cytosine units is approximately equal to that of guanine (%C ≈ %G).