concluded that, in DNA, adenine levels were equal to thymine levels and cytosine levels were equal to guanine levels.

According to Chargaff, the DNA from any cell of a species should exhibit a 1: 1 ratio of purine and pyrimidine bases and, more precisely, that the concentration of adenine is equivalent to the concentration of thymine, and the concentration of guanine is equivalent to the concentration of cytosine. This arrangement is observed in both the strands of the DNA, also known as the base pair rule.