Erwin Chargaff’s discovery that DNA contains equimolar amounts of guanine and cytosine and also equimolar amounts of adenine and thymine has come to be known as Chargaff’s rule:

G = C and A = T

(a) Does Chargaff’s rule imply that equal amounts of guanine and adenine are present in DNA? That is, does G = A?

(b) Does Chargaff’s rule imply that the sum of the purine residues equals the sum of the pyrimidine residues? That is, does A + G = C + T?

(c) Does Chargaff’s rule apply only to double-stranded DNA, or would it also apply to each individual strand if the double helical strand were separated into its two complementary strands?

Chargaff’s rule implies that total number of purine bases is equal to total number of pyrimidine bases, that is, number of guanine units is equal to cytosine units present and number of adenine units equal to thymine units. DNA composition is different in different species. Chargaff’s rule do not apply that number of adenine units are equal to number of guanine units.

In double stranded structure of DNA,

A = T = 1

G = C = 1

Percentage of C + G does not necessarily equal to percentage of A + T.

Sum of purine residues is equal to sum of pyrimidine residues. In a double stranded DNA, number of purines and pyrimidines exist in 1:1 ratio as purines bond with pyrimidines in DNA.

Chargaff’s rule apply only to double-stranded DNA. For each G in a strand, there exists a complementary C in the opposing strand but there is no relation between G and C in the same strand. Complementary base pairing is not applicable in case of RNA as it is single stranded, thus Chargaff’s rule is not applied to RNA.