This is the foundation for DNA replication the rule that states that purine adenine always pairs with the pyrimidine thymine and the pyrimidine cytosine always pairs with the purine guanine
Erwin Chargaff, a Columbia university biochemist analyzed the proportions of the purine (adenine and guanine) and pyrimidine (cytosine and thymine) bases in DNA.
Chargaff discovered that in DNA, the purines and pyrimidines are in a 1:1 ratio i.e the percentage of purines equal to that of pyrimidines.
He further concluded that the amount of adenine was approximately equal to the amount of thymine and that of cytosine was approximately equal to that of guanine.
Therefore, adenine always pairs with thymine whereas cytosine always pairs with guanine.
