MOLECULAR GENETICS 3

Search for the Genetic Material

The study of Genetics started with Gregor Mendel who introduced ‘factors’ for inheritance. 

Boveri-Sutton theory later gave the chromosomal theory of inheritance.

The concept of inheritance was understood at the level of chromosomes further when Morgan came up with the concept of linkage & recombination at chromosomal level.

It was taken a step forward in 1926 when experiments were being performed to understand Inheritance at a molecular level.

The search was for the molecule which acts as a genetic material.

Griffith’s Experiment

Griffith experimented with Streptococcus pneumoniae bacteria. 

This bacteria causes Pneumonia.

Two strains -- R-strain & S-strain.

• S strain
  • Smooth mucous polysaccharide coat
  • Resistant to immune system
  • Virulent
• R strain
• Lacks the coat
• Destroyed by immune system of the host
• Non-virulent

The experiment was performed in multiple steps.

1. S-strain (virulent) was injected into mouse. It was found that the mouse died of pneumonia

R-strain (non-virulent) was injected into mouse. It was found that the mouse remained alive

Heat killed S-strain (S-strain bacteria were killed by heating) was injected into mouse. It was found that the mouse remained alive

Heat killed S-strain & live R-strain were injected into mouse. It was found that the mouse died of pneumonia

Griffith found that Live S-strain bacteria could be recovered from the dead mouse (Step 4).

Griffith thus arrived at the following conclusion:

• Something caused bacteria to change from one type (R) to another type (S)
• Some ‘Transforming principle’, transferred from heat-killed S strain to R strain and transformed it virulent.

However, the biochemical nature of the ‘Transforming principle’ was still unknown.