Early Experiments in the Discovery of DNA As the Genetic Material
In 1928 Frederick Griffith opened the door to helping solve the DNA versus protein debate after noting some interesting results from experiments he was performing. He was exposing mice to bacteria that occurred in two forms. One form of the bacteria was not pathogenic, and the mice survived exposure to this strain of the bacteria. The other strain of the same bacterial pathogen was virulent, and caused death in the mice. Griffith killed some of the virulent bacteria through boiling and exposed the mice to the dead bacteria. The mice exposed to the killed bacteria were fine and survived. However, if the boiled and killed virulent bacterial strain was injected into a mouse along with the non pathogenic strain at the same time, the mouse would die. This interesting result caused Griffiths to realize that somehow the genetic factors of the dead virulent strain were transforming the non pathogenic alive bacteria into the deadly version and causing death in the mice.
Expounding on these discoveries made by Frederick Griffith in 1928, Oswald Avery, Colin MacLeod, and Maclyn McCarty performed their own revealing experiments in 1944. The experiment they designed was aimed at answering the question of which component from the boiled killed bacteria transformed the non pathogenic bacteria into the deadly strain. The experiment destroyed one component from the pathogenic strain of bacteria at a time and was added to the mice with the non pathogenic strain. When the polysaccharides (sugars), lipids (fats), RNA, or protein were destroyed in the virulent strain and added to the mice along with the non pathogenic strain the mouse still died. When the DNA from the pathogenic strain was destroyed and added with the non pathogenic strain, the mouse survived. This helped verify that the transforming agent was DNA. This discovery helped to show that the hereditary nature and information of genetics was contained in DNA. Many scientists of the time were not inclined to accept this as proof that DNA was the genetic material, and thought that protein was the genetic material due to the complexity of proteins.
The definitive proof that DNA was the information containing genetic material came with the experiments performed by Alfred Hershey and Martha Chase in 1952. The experiment involved the use of radioactive elements, bacteria, and phages (viruses that infect bacteria and transfer some of their genetic material to the bacterial cells in the process). Proteins contain the element Sulfur while DNA does not, and DNA contains phosphorous while proteins do not. This key difference between proteins and DNA was instrumental. Radioactive phosphorous was incorporated into phage DNA, while radioactive sulfur was incorporated into the phage proteins. The phages were then allowed to infect bacteria. Using differential centrifugation and radioactivity detection the experiment showed that the radioactive phosphorous ended up inside the bacterial cells, while the sulfur did not. This showed that the DNA was the only genetic component that entered the bacterial cells, and therefore the genetic material transforming the bacteria. This final experiment gave the scientific community the definitive evidence needed to finally confirm that DNA was the genetic material of organisms.