Chargaff’s rule states that DNA from any cell of any organism has a 1:1 ratio of pyrimidine and purine bases and, more specifically, that the amount of guanine, a purine base, is equal to cytosine, a pyrimidine base; and the amount of adenine, a purine base, is equal to thymine, a pyrimidine base.
So a base pair is composed of a pyrimidine base and a purine base. This pattern is found in both strands of DNA, and is responsible for the base-pairing rule, which states that adenine always pairs with thymine, and guanine always pairs with cytosine..
The nitrogen bases pair with each other by means of hydrogen bonds.
