What is Chargaff ‘s rule of base equivalence?

Deoxyribonucleic acid (DNA) is the genetic material found in the chromosomes of all animals and plants. It is made up of only four types of organic nitrogenous bases: adenine (A), guanine (G), thymine (T) and cytosine (C). Of these, A and G are the purines and T and C are the pyrimidines. Chargaff gave the base pairing rule or the rule of base equivalence which states that only one purine can combine with one pyrimidine. That means A can combine with T and G with C. Two purines or two pyrimidines cannot combine with each other; if they do so, there will be a sudden change in the characteristic of an organism. This sudden change is called mutation.