1. What is genetic material?

The research at the end of the 19th century had verified Mendelian inheritance and it was also believed that the genetic material is in the chromosome. However, scientists still didn't know the true features of the genetic material. In the early twentieth century, biologists believed that proteins carried genetic information. But the Griffith experiment with Streptococcus pneumoniae (1928), Avery, MacLeod and McCarty experiment (1944) on transforming principle and Hershey-Chase experiment (1952) on bacteriophage T2, confirms that DNA is genetic material. Genetic material is the material that determines the inherited characteristics of a functional organism. It has the following properties:

•  It must be stable

•  It must be capable of being expressed when needed

•  It must be capable of accurate replication

•  It must be transmitted from parent to progeny without change

 

2. The transforming principle

In the year 1928, Griffith used two types of strains of pneumonia-causing bacterium, Streptococcus pneumoniae, using mice for his experiment. One was S-type (smooth) strain having a polysaccharide coat and produces smooth, shiny colonies on a lab plate. The polysaccharide coat of S-type makes it resistant to the immune system of mice. The other strain, R-type (rough) strain, lacks the coat and produces colonies that look rough and irregular. The R-type lacks the polysaccharide coat and thus it will be destroyed by the immune system of the host. Griffith discovered that there was 'something' that causes the convertion of the R-strain to virulent S- strain.

In the first stage of the Griffith's experiment, he showed that when mouse was injected with S-type strain, mouse died of pneumonia but when injected with R-type strain, mouse lived. The next stage showed that if heat-killed S-type strain was injected to mouse, all mice lived, and this result suggested that the bacteria had been rendered ineffective. The attractive results came with the third part of the experiment, where mice were injected with a mixture of heat-killed S-type strain and non-virulent R-type strains; interestingly all mice developed pneumonia and died. (Figure 21.1). In their blood, Griffith found live bacteria of the deadly S- type. The S strain extract somehow had "transformed" the R- strain bacteria to S- form.

Figure 21.1: Griffith's experiment discovering the "transforming principle" in Pneumococcus bacteria