Scientists dealing in molecular biology proved with the help of experiments that DNA is a hereditory material. There are two main evidences viz –
(A) Griffith’s experiment:
It was done in 1928 by Frederick Griffith. It was one of the experiment showing that bacteria can get DNA through a process called transformation. He used two strains of bacteria, called Diplococcus pheumonae viz. – type II-R (rough or non virulent) and type III – S (Smooth or virulent). These bacteria infect rats & human beings.
The strain III- S is pathogenic and the strain II – R is non- pathogenic. The strain III- S covers itself with a polysaccharide capsule that protects it form the host immune system. This means that the host cell will die. The strain II-R does not have the protective capsule and it is killed by the host immune system.
In this experiment, the bacteria of strain III – S were killed by heat and their remains were added to strain II-R bacteria. The killed strain III-S and strain II-R harmed the mice but the blend of the two was able to kill mice. Griffith was also able to get both live II-R and III- S strains from the blood of these dead mice. He concluded that the type II-R had been transformed into the lethal III- S strain by transforming principle. Griffith observed-
- Strain III- S, when injected kill the mice.
- Strain II- R, when injected, the mice remained live (unaffected).
- Killed strain III- S when injected, the mice remained live (unaffected).
- When both the killed strain S – III and strain II – R were injected, they killed the mice.
Griffith concluded that while the bacteria of strain III-R had been killed, the DNA had survived the heating process and was taken up by the strain II-R. Armed with DNA of strain III-S, the bacteria of strain II-R developed the capsule and get protected from the host’s immune system and could kill the mice.
Hence, it can be concluded that DNA is the hereditory material
