In 1928, Frederick Griffith, a British medical officer, identified DNA as the source of genetic material as he experimented with the pneumonia causing bacterium. He observed that the living R-bacteria acquired the ability to form polysaccharide capsules from the dead S-bacteria. He termed this phenomenon as transformation and the transforming principle as the responsible agent (Russell 2011). Later, in the 1930s, Oswald Avery, together with his colleagues, Colin Macleod and McCarty, confirmed Fredrick’s hypothesis. They performed an experiment meant to identify the chemical configuration of the transforming principle observed in Fredrick’s experiment. They treated the macromolecules extracted from the S-bacteria with enzymes to obtain the three main molecules of the hereditary material. They observed that both the destruction of the proteins and RNA did not interfere in any way with the S-bacteria transforming the R-bacteria into virulent S-bacteria (Hartl 2011). However, with the destruction of the DNA, the transformation halted and no smooth colonies were observed. In 1944, Avery and his colleagues concluded that DNA was the transforming principle highlighted by Fredrick. However, their results were not completely accepted with some biologists arguing that some proteins survived the enzyme treatment and were responsible for the transformation phenomenon.
In 1952, Alfred Hershey and Martin Chase conducted as series of experiments that eliminate any doubts that the DNA formed the biochemical of heredity. The studied the infection of the bacterium Escherichia coli by the virus Bacterio-phage T2.E (Wheelis 2008). This virus consists of a DNA core surrounded by proteins. They hypothesized that one of the two molecules was the genetic material that facilitated the virus infection and replication in the host cell. They used the radioactive labeling approach and used the label as a tag to each molecule through the phage life cycle. They established that the labeled DNA, but not the labeled protein, entered the cell and appeared in the progeny phages (Solomon et al. 2008). Thus, they conclusively established that the genetic material is DNA and not protein. In 1953, James Watson and Francis Crick demonstrated how the various DNA components fit together by creating a 3-D model of the double helix (Wilmut 2001). They demonstrated that DNA was composed of two polynucleotide chains held together by hydrogen bonds between adenine and thymine and between guanine and cytosine (Watson & Watson 1991).