Erwin Chargaff’s rules: Chargaff along with his colleagues, performed base composition studies and put forward certain generalizations for double stranded DNA, called as Chargaff’s rule (Not applicable for single stranded DNA or RNA).
i) Purines and pyrimidines occur in equal amounts.
ii) Purines found in DNA are adenine and guanine. Pyrimidines of DNA are thymine and cytosine.
A + G = T + C
iii)  $\frac{A+G}{T+C} = 1$ this value is constant for all species.
iv) The base ratio $\frac{A+T}{G+C} = 1$ is specific for a species. It is used to identify the species. It is less than one in prokaryotes, e.g., E. coli = 0.92 and more than one in eukaryotes, e.g., Human = 1.52.
v) Sugar deoxyribose and phosphate residues occur in equal number.
vi) Purine adenine is equimolar with pyrimidine thymine.
vii) Purine guanine is equimolar with pyrimidine cytosine.