 The discovery of the structure of DNA is one of the greatest events in the history of science. This event not only gave momentum to many existing disciplines but it also gave birth to some new disciplines in the field of biological sciences. A number of scientists, they worked to elucidate the structure of DNA and but only few, the work of only few workers is mentionable and Erwin Shargov is one of those. Erwin Shargov with his colleagues, they provided the most important clue to the structure of DNA and the work of Shargov gave some conclusions and these conclusions are called Shargov rules. The first rule states that the base composition of DNA varies from one species to another. It means if we isolate DNA from one species, the base composition of DNA is this DNA is different from the DNA isolated from another species. What is base composition? Base composition means the particular sequence of the four nucleotides, the four types of different nucleotides they arrange in a particular sequence and this sequence is called the base composition. The second point states that the DNA isolated from different tissues of the same species have the same base composition. It means if the DNA is isolated from an organism, from the different parts of that organism, maybe from the skin, from the hair, from the nail or from some any other organ, the base composition of all these DNAs isolated from different parts of that organism are same. The third point, the third point is the base composition of DNA of a given species. Doesn't change with the organism's age, nutritional state or changing environment. It means it doesn't happen that the base composition is different when the organism is young and it is different when it is old. Similarly, if the organism is hungry or is starved, its base composition will be different. And thirdly, if the environment or weather is changing, base composition doesn't change. The fourth point is which is maybe the most important point and which gave a most important clue to Watson and Crick who finally described the structure of DNA. And this rule states that the number of adenosine in DNA is equal to the number of thymidine and similarly the number of guanosine residues in a DNA molecule is equal to the number of cytidine residues. That is A is always equal to T and G is always equal to C. It means that the sum of pureins equals to the sum of pyrimidines because you know A and G are pureins and T and C are pyrimidines. So, pureins are equal to pyrimidines in a DNA molecule.