Question

(a) Explain the experiment performed by Griffith on Streptococcus pneumonia. What did he conclude from this experiment?
(b)Name the three scientists who followed up Griffith's experiments.
(c)What did they conclude and how?

Answer 1

Hint: (a) Griffith experiment was a stepping stone for the invention of genetic material. Frederick Griffith experiments were conducted with Streptococcus pneumonia injected in mice.
(b) The three reported that DNA is the substance that causes bacterial transformation, in an era when it had been widely believed that it had been proteins that served the function of carrying genetic information.
(c) They conclude that protein-digesting enzymes (proteases) and RNA-digesting enzymes (RNases) didn't affect transformation.

Complete answer:
(a) During the experiment, Griffith cultured Streptococcus pneumonia bacteria which showed two patterns of growth. One culture plate was made up of smooth shiny colonies (S) while the other consisted of rough colonies (R). The difference was due to the appearance of the mucous coat in S strain bacteria, whereas the R strain bacteria lacked them.
Experiment: In Griffith’s experiment, he injected both S and R strains into mice. The one which was infected with the S strain grew pneumonia and died while those infected with the R strain remains alive.
In the second stage, Griffith heat-killed the S strain bacteria and injected it into mice, but the mice remained alive. Then, he mixed the heat-killed S with live R strains. This mixture was injected into mice and they died. Additionally, he found living S strain bacteria in dead mice.
Conclusion: supported by the observation, Griffith concluded that R strain bacteria had been transformed by S strain bacteria. The R strain inherited a particular ‘transforming principle’ from the heat-killed S strain bacteria that built them virulently. And he concluded this transforming principle as genetic material.
(b)Griffith's experiments were followed by - Oswald Avery, Colin MacLeod, and Maclyn McCarty (1933-44).
(c) They concluded that the transformation was caused by DNA. They came to the present conclusion on the idea of the very fact that Protein-digesting enzymes and RNA-digesting enzymes didn't affect transformation. The transformation was inhibited by digestion with the DNA-digesting enzyme (DNase).

Note: Griffith’s experiment was a turning point towards the discovery of hereditary material. However, it failed to explain the biochemistry of genetic material. Therefore, a group of three scientists named Oswald Avery, Colin MacLeod, and Maclyn McCarty carry on with the Griffith experiment in search of the biochemical identity of the hereditary material. Their discovery updates the theory of protein as genetic material to DNA as genetic material.