Question

Describe the series of experiments of $F$. Griffith. Comment on the significance of the results obtained.

Answer 1

Griffith's experiment was an experiment done in 1928 by Frederick Griffith. It was one of the first experiments showing that bacteria can get DNA through a process called transformation.

1. Griffith used two strains of Streptococcus pneumoniae. These bacteria infect mice. He used S (smooth) and R (rough) strain.
   
2. The S strain covers itself with a polysaccharide capsule that protects it from the host's immune system. This means that the host will die. The R strain does not have that protective shield around it and is killed by the host's immune system.
   
3. In this experiment, bacteria from the S strain were killed by heat, and their remains were added to R strain bacteria. While neither harmed the mice on their own, the blend of the two was able to kill mice.
   
4. Griffith was also able to get both live R and live S strains of S. pneumoniae from the blood of these dead mice.
   
5. He concluded that the type R had been "transformed" into the lethal S strain by a "transforming principle" that was somehow part of the dead S strain bacteria.

Today, it is known that the "transforming principle" Griffith reported is the DNA of the S strain bacteria. While the bacteria had been killed, the DNA had survived the heating process and was taken up by the R strain bacteria. The S strain DNA contains the genes that form the shielding polysaccharide part from attack. Armed with this gene, the former R strain bacteria were now protected from the host's immune system and could kill the host. The work prompted many other biologists to seek deeper into understanding the science behind heredity.The exact nature of the transforming principle was confirmed by scientist Avery, McLeod and McCarty and by Hershey and Chase.