Transforming Principle

Heredity

In the 20th century, we already knew heredity : the physical characteristics of a living being could be transmitted from generation to generation thanks to hereditary factors. However, we didn't know these factors !

What could possibly transmit the characters of a relative to his or her descendants ?

Archibal Garrod, in 1909, was the first to make the link between hereditary diseases and proteins. He demonstrated that some hereditary diseases are due to the absence of certain enzymes in the individual. Some abnormalities could also be due to hereditary abnormalities in an enzyme !

But why could an individual form an abnormal enzyme ? And how could he transmit it to his or her descendants ?

The experience of Griffith :

In 1928, Griffith was working on a bacterium that could cause death in humans : Streptococcus pneumoniae. First, he noticed that, in his crops, there was sometimes a different and non-lethal variety of the bacterium. So when you injected the normal variety (in blue) into a mouse, it died, whereas if you injected the new variety (in red), it didn’t die.

Griffith then had the idea of injecting mice with a mixture : to mix the new and harmless variety with dead bacteria of the normal variety. After injection on a mouse, he sees that this mixture is lethal. In addition, mice killed by this injection contain deadly bacteria !! We conclude that the avirulent bacteria have transformed into virulent bacteria !! In contact with killed virulent bacteria, avirulent bacteria become deadly ! It is then interpreted that the avirulent bacteria that didn't have enzymes, was harmless, received an enzyme from the virulent bacteria, which made them lethal.

But how can one thing pass from one bacterium to another ? How can one bacterium transmit and give its information to another ?

Avery-MacLeod-McCarty experience :

In the 1940s, three researchers (Avery, MacLeod and McCarty) will demonstrate that the transforming substance that passes from dead virulent bacteria to live avirulentes is DNA.

Using the Griffith experiment, they treat dead virulent bacteria with a digestive enzyme before mixing them with avirulent bacteria.

  • if the enzyme used is a protease (enzyme that digests the protein), the non-virulent bacteria still transform into virulent bacteria.

  • if the enzyme used is a DNase (enzyme that digests DNA), then the transformation does not take place !

The conclusion is that it’s the DNA that causes the transformation. So it’s the DNA that transmits the genetic information.