Molecular basis of inheritance

Experiment for the search of genetic material

Griffith experiment or Transforming principle

Griffith experiment was a stepping stone for the discovery of genetic material. Frederick Griffith experiments were conducted with Streptococcus pneumoniae. He took two strains of the bacteria.

1. Virulent or S-III or Smooth - This bacteria has an extra protective mucous coat due to which it is smooth in appearance and causes disease.

2. Avirulent or R-II or Rough - It does not have the extra protective layer due to which it is rough in appearance and do not cause disease.

He did the following experiment

• When he injected avirulent strain to mouse , it survives.

• When he injected virulent strain to mouse, it dies due to pneumonia.

• When he heat killed the virulent strain and injected in mouse, it survives.

• When he mixed heat killed virulent with avirulent and injected to mouse , he thought it would survive, but it died within few days.

From this experiment he concluded that there are some factors in the heat killed virulent that transforms avirulent to virulent that causes disease.

Biochemical basis of Griffith 's experiment

Griffith experiment was a turning point towards the discovery of hereditary material. However, it failed to explain the biochemistry of genetic material. Hence, a group of scientists, Oswald Avery, Colin MacLeod and Maclyn McCarty continued the Griffith experiment in search of biochemical nature of the hereditary material. Their discovery revised the concept of protein as genetic material to DNA as genetic material.

They took large amount of virulent strain of Streptococcus pneumoniae . They heat killed it and centrifused at 1000 rpm. The solution was filtered and the filterate containg carbohydrates, proteins, RNA and DNA was added to avirulent strain, transformation occurs i.e. avirulent gradually get converted into virulent. They concluded that the transforming particles lie in the filterate.

To a part of filterate small amount of enzyme protease was added which digest the protein, when added to avirulent it get converted into virulent. Hence protein is not the transforming particle .

To another part RNase was added which digest RNA when added to avirulent it get converted into virulent. Hence RNA is not the genetic material.

To remaining part enzyme DNase was added which digest DNA , when added to avirulent no transformation occurs. Hence they concluded that DNA is the genetic material.