To search the genetic material Frederick Griffith transforming experiment on mice

 

• The postulation of law of inheritance by Mendel triggered the search for Genetic material.

• No doubt researchers opted  Mendel law of inheritance as base   for the search of  genetic material.

• In a series of experiments that were conducted by various scientists, Griffith experiments was one of the early experiments to prove the fact of genetic material.

• The Griffith experiment was a stable step for the discovery of genetic material.

Griffith Transforming Experiment 

• In 1928 Frederick Griffith  conducted a series of experiments by using bacteria  Streptococcus pneumonia responsible for infection of pneumonia.

• During the experiment, Griffith cultured Streptococcus pneumoniae bacteria in the lab.

• He cultured two strains of bacteria. One strain of bacteria is smooth colony or S strain or virulent type and other bacteria is rough  colony or R strain or non virulent.

• The major difference between the both strains of bacteria is that smooth colony bacteria have a coat of mucopolysaccharides whereas this coat remains absent in rough colony bacteria.

Read Hershey and Chase Experiment

• The Griffith experiments can be summarised in four steps -

• Step -1 Griffith injected R strain bacteria into the body of mice and mice were alive.

• Step -2 Griffith injected S strain bacteria into the body of mice and mice died due to pneumonia.

• Step -3  This time Griffith injected heat killed  S strain bacteria into the body of mice and mice was alive.

• Step -4  in this step, he mixed the heat-killed S and R strains. This mixture was injected into the body of mice and mice was died. In addition, he found living S strain bacteria in dead mice

• Griffith concluded  on the basis of the above observation that - The mucopolysaccharides coat of S strain is responsible for pneumonia  therefore due to insertion of S strain bacteria mice died.

• R strain was devoid of mucopolysaccharides coat so mice were alive when R strain had been inserted into them.

• When  the mixture of heat-killed S strain bacteria  and R strains was injected into the body of mice and mice was died because somehow R strain bacteria had been transformed by S strain bacteria. 

• The R strain inherited some ‘transforming principle’ from the heat-killed S strain bacteria which made them virulent.

• But Griffith could not concluded that What was the reason for transforming of R Strain by S strain.

• The Griffith experiment was a landmark approach  towards the discovery of hereditary material.

• However, the Griffith experiment could not  explain the biochemistry of genetic material but Griffith introduced a path to search the genetic material.

Read Meselson and Stahl Experiment

• Later on Oswald Avery, Colin MacLeod and Maclyn McCarty from 1933 to 1944 reworked the Griffith experiment to explore the  biochemical nature of the hereditary material.

• These scientists extracted  proteins, DNA, RNA and other biomolecules from the heat-killed S strain bacteria.

• When they applied three enzymes named protease RNases and DNases. These enzymes digest protein, RNA and DNA respectively present in cell.

• They observed that  transformation in mice was not affected by the protease and RNases but DNase affected transformation and they concluded DNA as genetic material but all the biologists did not accept this conclusion.