The Transforming Principle of Pneumococcus

Avery, Macheod and Mc Carty discovered that the Transforming Principle of Pneumococcus is DNA.

Background

The pneumococcus bacteria causes pneumonia, sepsis and meningitis. In 1928, Frederick Griffith discovered that the pneumococcus bacteria had two strains - the S-strain and the R-strain. The S-strain was virulent, meaning it caused disease, while the R-strain was non-virulent. Griffith discovered that when heat-killed S-strain bacteria were injected into mice along with live R-strain bacteria, the mice died. This led to the discovery of the Transforming Principle - some substance in the dead S-strain bacteria was transforming the live R-strain bacteria into virulent ones.

Experiment

Avery, Macheod and Mc Carty wanted to find out what the Transforming Principle was made of. They isolated the substance from the S-strain bacteria and treated it with enzymes that destroyed different types of molecules - protein, RNA and DNA. They then mixed each treated substance with R-strain bacteria and checked whether the bacteria were transformed into virulent ones.

Results

The results showed that when DNA was destroyed, the R-strain bacteria did not transform into virulent ones. However, when protein and RNA were destroyed, the R-strain bacteria still transformed into virulent ones. This led to the conclusion that the Transforming Principle was DNA.

Significance

This discovery was significant because it showed that DNA was the genetic material responsible for inheritance and variation. It also paved the way for the discovery of the structure of DNA by Watson and Crick in 1953.

DNA