In this article we will discuss about the Chargaff-equivalence rule for DNA.
By 1948, a chemist Erwin Chargaff started using paper chromatography to analyse the base composition of DNA from a number of studies. In 1950, Chargaff discovered that in the DNA of different types of organisms the total amount of purines is equal to the total amount of pyrimidines i.e. the total number of A is equal to the total number of T (A – T), and the total number of G is equal to the total number of C (G – C).
It means that A/T = G/C i.e. A + T/G + C = 1. In the DNA molecules isolated from several organisms regularity exists in the base composition.
The DNA molecule of each species comprises of base composition which is not influenced either by environmental conditions or growth stages or age. The molar ratio i.e. [A] + [T]/[G] + [C] represents a characteristic composition of DNA of each species.
However, in higher plants and animals A – T composition was found generally high and G – C content low, whereas the DNA molecules isolated from lower plants and animals, and bacteria and viruses was generally rich in G – C and poor in A – T contents (Table 5.2).
Table 5.2 : Relative amount of nitrogenous bases in DNA isolated from different organisms.
The two closely related species will have very similar molar % G+C values and vice versa. Thus, the use of base composition has much significance in establishing relationship between two species and in taxonomy and phylogeny of species.