Erwin Chargaff, in late 1940s, quantitatively analysed the DNA hydrolysates from different species. The important conclusions drawn by him are the following:

1. The sum of purines is equal to the sum of pyrimidines; that is, pu/py = 1. In other words, A + G = T + C.
2. The ratio of adenine to thymine is also one, A/T = 1.
3. The ratio of guanine to cytosine is also one, G/C = 1.
4. Bases with 6-amino groups are equal to bases with 6-keto groups; that is, A + C = G + T.
5. The ratio of A + T/G + C, known as dissymmetry ratio, varies greatly from one species of DNA to the other and is characteristic of that species. When the dissymmetry ratio exceeds one, such a DNA is called AT type. When the value is less than one, such a DNA is designated as GC type. In most animals, the value ranges from 1.3 to 3.2 and in higher plants, from 1.1 to 1.7. DNA composition of human liver is presented in Table 5.4.

Table 5.4 DNA Composition of Human Liver

The double helical structure of DNA derives its strength from Chargaff’s rule.

Single-stranded DNA, which is usually single stranded, does not obey Chargaff’s rule. However, double-stranded RNA, which is the genetic material in certain viruses, satisfies Chargaff’s rule.