DNA : as a genetic material

Many experiments were carried out to identify the genetic material. It was discovered that. DNA is a genetic material through the experiments using bacteria and viruses.

Griffith Experiment:

The first series of experiments were performed by a British physician F. Griffith in 1928 using the bacterium Diplococcus pneumoniae, which causes pneumonia in mammals.

There are two types of strains S-type and R-type. The experiment was conducted in following manner.

  1. S-type, is capsulated and smooth. When S-type of bacteria were injected into healthy mice, the mice developed pneumonia and died. Thus, S-type is virulent or pathogenic.
  2. R-type is non-capsulated and rough.When R-type of bacteria wee injected into healthy mice, they did not cause pneumonia. Thus, R-type is avirulent or non-pathogenic.
  3. Heat killed S-type of bacteria were injected into healthy mice, they did not cause disease and the mice remained healthy. It is avirulent.
  4. Mixture of heat killed S-type of bacteria and living R-type bacteria was injected into healthy mice, the mice developed pneumonia and died. It is virulent.

The blood sample from dead mice was cultured, colonies of living S-type were obtained.

Conclusion from Experiment [step 4]:

The R-type of bacteria must have picked up 'something' from the surrounding medium that contains heat killed S-type, and got changed to S-type.

Therefore, this change is due to the phenomenon other than mutation, which he called Transformation. He named that 'something' as transforming

principle.

Later it was discovered that the transforming principle was DNA.

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Griffith's discovered transformation, he first showed gene action.

  1. The transforming principle was picked up by the living R-type from the surrounding medium that contained heat killed S-type.
  2. Enabled the R-type to synthesize a smooth polysaccharide coat (capsule) and became virulent
  3. This principle was passed to the next generations and colonies of S-type were obtained.

Therefore, this transforming principle must be the genetic material.

Avery, Macleod and McCarthy Experiment

They purified DNA. RNA. proteins, and other materials from heat killed S-type and mixed with R-type to see which ones could transform living R-type to S-type.

R-type mixed with DNA were transformed into S bacteria.

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Mixture of S-type is heat-killed and contains only biomolecules from that strain.