The first clear and published description of Deoxyribonucleic acid (DNA) was done 1869 by Friedrich Miescher characterizing DNA as an acid precipitate material formed from the nucleic acid with unusual proportions of nitrogen and phosphorus. Four years before that, Mendel’s work which established that heredity was controlled by factors and chromosomes was published. However, it took 80 years to firmly establish the role of DNA as the genetic material in most organisms when Fredrick Griffith carried out experiments with smooth (S) virulent strain Streptococcus Pneumonia and the rough (R) non-virulent strain of the same bacteria. In 1928, Griffith carried out the first unambiguous experiment which suggested that protein was not a genetic material.

He observed that the mice died after injecting them with the virulent S bacteria. He later killed the virulent bacteria by heating them but after injecting the mice with a mixture of the killed S variants and the live R variants, they died. Eager to establish what happened, he isolated the bacteria from the dead mice and found they were of S variety. This showed that the bacteria had been transformed from the rough non-virulent to smooth virulent version. Since Griffith knew heat denatures proteins, he concluded that the carrier of the genetic material was something else (Omoto & Lurquin, 2004). However, Griffith’s experiments by themselves did not establish DNA is the genetic material. As a result, Oswald Avery, Colin MacLeod and MacCarty demonstrated in 1944 that DNA is the transforming material in the Griffith’s experiments. They showed that the transforming principle in the Griffith’s study can be destroyed by pancreatic deoxyribonuclease, an enzyme that hydrolyses DNA. Their experiments ruled out lipids, polysaccharides, RNA and proteins as the transforming elements after adding Griffith’s mixture with pancreatic ribonuclease and other enzymes which destroyed proteins.

To confirm and provide evidence that DNA is the genetic material Alfred Hershey and Martha Chase carried out a series of experiments in 1952 using T2 bacteriophage and bacteria. They specifically used T2 phage virus because it had a simple structure of an outer core and inner component after it had shown using electron microscopy. The scientists had already established that the phage infect other cells by anchoring the other shell on the cell surface and deposit their inner components into the victim’s cell. However, they were eager to establish whether the inner component was a protein or a DNA component (Hartl et al. 2006).

Hence, they incorporated radioactive ³²P on the DNA component and ³⁵S on the protein components on the infecting phage and used radioactive tracers to examine the infected bacteria. They observed that the DNA component was indeed introduced into the infected bacteria supporting the hypothesis the material which infected the bacteria was DNA.

Clinical Psychologist
Beyhan Perim Seçmen


Hartl, D. L., Jones, E. W., & Lozovsky, E. R. (2006). Study guide and solutions manual to accompany
Essential genetics : a genomics perspective, fourth edition (4th Ed.). Sudbury, Mass.: Jones and
Bartlett Publishers.

Omoto, C. K., & Lurquin, P. F. (2004). Genes and DNA: a beginner’s guide to genetics and its
applications. New York: Columbia University Press.