D.N.A.(Deoxyribonucleic acid) is the genetic material
found in the chromosomes of all animals and plants.
It is made up of only 4 types of organic nitrogenous bases:
A(adenine),G(guanine),T(thymine) and C(cytosine).
Of these, A and G are the purines and T and C are the
pyrimidines. Chargaff said that only one purine can
combine with one pyrimidine(A 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.