Chargaff basepair rule

A few years before Watson and Crick proposed their essentially correct threedimensionalstructure of DNA as a double helix, Erwin Chargaff developed a chemical technique to measure the amount of each base present in DNA. As we describe this technique, we will let the molar concentration of any base be represented by the symbol for the base in square brackets; forexample, [A] denotes the molar concentration
of adenine. Chargaff used his technique to measure the [A], [T], [G], and [C] content of the DNA from a variety ofsources. He found that the base composition of the DNA, defined as the percent G C, differs among species but is constant in all cells of an organism and within a species.
Chargaff also observed certain regular relationships among the molar concentrations of the different bases. These relationships are now called Chargaff’s rules:
• The amount of adenine equals that of thymine: [A] [T].
• The amount of guanine equals that of cytosine: [G] [C].
• The amount of the purine bases equals that of the pyrimidine bases: [A] [G] [T] [C].
Although the chemical basis of these observations was not known at the time, one of the appealing features of the Watson–Crick structure of paired complementary strands was that it explained Chargaff’s rules. Because A is always paired with T in doublestranded DNA, it must follow that [A] [T]. Similarly, because G is paired with C, we know that [G] [C]. The third rule follows by addition of the other two: [A] [G]
[T] [C].