One unique character of DNA is its unity and yet infinitesimal variety. The DNA structure mentioned above is uniform in all organisms, whether it is man or virus. At the same time it is also true that DNA is the genetic material that decides the characters of individual organisms.
Then how does DNA decide the individuality? The answer lies in the base pair sequence. The combination of A/T, G/C is uniform, but their sequence is not; for instance in one individual it may be AT, GC, AT, AT, AT, in another- GC, GC, AT, GC, AT etc. There is no limit to the variety in which base pair sequences are arranged. This is the secret of individual differences. In no two individuals, the base pair sequence is same.