Genes are the building blocks of heredity. All living things inherit the genetic information specifying their structure and function from their parents. Likewise, all cells exist from pre-existing cells and therefore the genetic information has to be replicated and transmitted from one generation to the other at each cell division. (Cooper, 2000)
The first evidence leading to the identification of DNA as the genetic material came from studies in bacteria. It dates back to 1928 while Fredrick Griffith was trying to find a vaccine against Pneumonia. He had two strains of the bacterium, one pathogenic and one non-pathogenic. He found that when he killed the pathogenic bacteria with heat and then mixed the remains with living bacteria of the non-pathogenic strain, some of the living cells became pathogenic. In addition, the living cells inherited the new trait of pathogenicity by all the descendants of the transformed bacteria. It had become clear that some chemical component of the dead pathogenic cells caused this hereditable change even though the identity of the substance was not known. Griffiths work went on to inspire the work of Avery, Maclyn McCarty, and MacLeod. They later went on to identify DNA as the transforming substance, however scientists were still sceptical.
Further evidence that DNA is the genetic material came from Erwin Chargaff. It was known that DNA was a polynucleotide that has three components: a nitrogenous base, a phosphate group and a pentose sugar called deoxyribose. The nitrogenous base could be Adenine (A), Thymine (T), Cytosine (C) and Guanine (G). He then analysed the base composition in a number of organisms and found that it varies from one species to another. He also found that the percentage of A and T bases were roughly equal, as were those of C and G bases. This was unexplained until the discovery of the double helix in 1953 by James Watson and Francis Crick. They used an X-ray image that was produced by Rosalind Franklin. This image revealed that DNA was helical and made up of two strands. Chargaff’s rule of base equivalence was now explained.