Frederick Griffith‘s experiment – The Hereditary Material

Which is the hereditary material? Until WWII many biologists believed proteins as the main hereditary material because of its great diverse amino acid sequences, linking the diversity of the genes directly with the diversity of proteins. Whereas, considering DNA as an un-credible hereditary material. It was during the mid-ninetieth century, the work of Gregor Mendel, the principle of inheritance, showed that the traits of a person are determined by some “factors” that he/she inherits. It is these “factors”, which are transferred from one generation to another. Based on Mendel’s study, many including Walter Sutton, Thomas Hunt Morgan narrowed and established Mendel’s heritable factors to be likely present in chromosomes located in the nucleus of most cells.

But the question which still remained was what material is responsible for inheritance? And how scientists did realize that it is DNA, which is the hereditary material. One such experiment among many was Griffith’s experiment in 1928 – identifying the process of transformation.


In 1928, British bacteriologist Frederick Griffith demonstrated a process we know as “transformation” – a process in which the bacteria are capable of transferring genetic information. Griffith was studying on experiments to develop a vaccine as pneumonia was causing a major threat to much life. He conducted experiments with mice and Streptococcus pneumonia bacteria; he used two strains of this bacteria, a virulent Type III- S (smooth) strain, the abbreviation “S” is to signify the smooth texture of that specific strain of bacterial colonies, these S- strain were having polysaccharide capsule on the surface, providing protection from mice immune system and thus virulent, whereas, in a harmless Type II- R (rough) strain, the abbreviation “R” is to signify the rough texture of that specific strain of bacterial colonies, were devoid of the polysaccharide capsule ( providing protection) making it non-virulent.

Griffith’s experiment:

Griffith experiment
by Madeleine Price Ball (CC0/Universal Public Domain

In a series of experiments, Griffith’s experiment involves injecting the two strains (S-strain and R-strain) into mice. As expected and as mentioned above about the bacterial strains, the Type III-S strain covered with polysaccharide capsule, protecting it from the host’s immune system and thus the mice injected with S-strain developed pneumonia and died, while the R strain, devoid of the protective capsule, is defeated by the host’s immune system and no pneumonia was developed by the mice.

In the next stage, Griffith injected the mice with S-strain bacteria that were heated in high temperature, also called heat-killed S bacteria. Here too as expected, the heat caused the death of the S-strain and thus did not cause pneumonia in the mice. The turning point of the experiment was about to come when he mixed the heat-killed S-strains and live R-strains. When he injected the mixture of the strains into the mice, he saw surprising results; not only the mice developed pneumonia and died but was detected with live S-strains in its blood sample.

Griffith concluded that the R-strain bacteria must have been transformed by the S-strain, what he called a “transforming principle”, allowing them to “transform” into smooth-coated bacteria having polysaccharide capsules cover on its surface and thus virulent. The above result basically concluded that the transformation factor must have contained the genetic material.


Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000. DNA: The genetic material. Available from:

Griffith, Fred. “The Significance of Pneumococcal Types.” The Journal of Hygiene 27.2 (1928): 113–159. Print

O’Connor, C. (2008) Isolating hereditary material: Frederick Griffith, Oswald Avery, Alfred Hershey, and Martha Chase. Nature Education 1(1):105

1944: DNA is “Transforming Principle”, National Human Genome Research Institute

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Frederick Griffith‘s experiment – The Hereditary Material

  1. What was the transforming material of the bacterial cells?
    a) Protein   b) DNA      c) RNA       d) Lipids
  2. In Griffith’s experiment, it was protein from S strain bacteria that caused the R strain bacteria to transform.
    a) True b) False  

  3. Against which pathogenic organism, Frederick Griffith was attempting to develop a vaccine?
    a) Bacillus anthracis      b) Streptococcus pneumonia 
    c) Pseudomonas            d) Moraxella
  4. How did Frederick Griffith prove that some ‘transforming material’ is responsible for the transformation of the non-virulent strains of bacteria into the virulent form?
  5. What killed the mice in Griffith’s experiment?
    a) heat-killed S strain bacteria killed the mice

    b) Mixture of heat-killed S strain bacteria and R strain bacteria to kill the mice.
    c) S strain bacteria killed the mice
    d) Both (b) and (C)
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