The most conclusive evidences in support of DNA as the genetic material comes from different avenues of approach on microorganisms – transformation of bacteria and transduction involving bacteriophages.
1. Griffith’s Experiment and Bacterial Transformation:
In 1928, Frederick Griffith, an English bacteriologist conducted transformation experiments on two strains of bacterium, Streptococcus pneumoniae (Diplococcus).
The two strains are as follows:
a. Smooth (S) or Capsulated Type:
The smooth strain has a capsule composed of polysaccharide (a polymer of glucose and glucuronic acid). This type of strain is virulent and causes pneumonia. The capsule is believed to protect the bacterium from attack by the immune system of the host.
b. Rough (R) or Non-Capsulated Type:
In this strain the capsule is absent. These bacteria are non-virulent and do not produce pneumonia.
The S and the R type occur in several subtypes and are represented as SI, SII, SIII, etc. and RI, RII, RIII, etc. respectively. These subtypes differ from each other in the type of antigens they produce. Griffith injected mice with both forms of the bacteria and obtained the results shown in Table 1.
In the fourth experiment, when post mortem was conducted on the dead mice, live capsulated forms were obtained. Griffith concluded that some substance passed from the heat killed capsulated forms to the live non-capsulated forms and made them virulent. However, the nature of this transforming principle was not known to Griffith.
After ten years of experimentation, in 1944 Avery, McLeod and McCarty identified the nature of the transforming principle. They separated the extract of the smooth virulent bacteria into protein, DNA and carbohydrate fractions. Each fraction was separately added to the culture of live rough bacteria and the following results were obtained (Table 2).
The culture to which DNA was added could produce smooth bacteria. This proved that DNA was the transforming agent. Further proof was provided when the enzyme, deoxyribonuclease was added to the DNA and then if the digested DNA was added, transformation did not occur. This experiment confirmed that DNA is the genetic material.
The phenomenon of transferring characters of one strain to another by using a DNA extract of the former strain is known as transformation or Griffith effect. Transformation is a permanent inheritable change produced in one strain of bacteria by a substance which is isolated from another strain of the same kind. The substance is known as the transforming substance or agent.
2. Transduction or Blender Experiment:
In the 1940s, viruses were the major experimental material. Viruses have a simple structure consisting of a protein coat surrounding a DNA or RNA molecule. In 1952, Hershey and Chase conducted the blender experiment with T2 bacteriophage, which is a virus that specifically attacks bacterial cells i.e. Escherichia coli.
The phage multiplies rapidly inside the E.coli cells in a very short period of time. Hershey and Chase demonstrated that only DNA of the phage enters the bacterial cell and, therefore, contains necessary genetic information for the assembly of new phage particles.
Hershey and Chase used radioactive isotopes to label the DNA and the protein of the virus. The phage protein contains sulphur, while the DNA contains phosphorus. They cultured the bacterium with the phage in a medium containing radioactive sulphur, 35S or radioactive phosphorus 32P. The phage protein becomes labelled with 35S and DNA with 32P.
The labelled phages were allowed to infect normally cultured bacteria in separate experiments. The cells were agitated in a blender. This stripped of the bacterial cells from the bacterial walls. The mixture was then incubated and then centrifuged. Radioactivity was then observed in the culture.
In the first set of bacterial cells, radioactivity was observed outside the cells in the supernatant, while in the second set, radioactivity was observed inside the cells. This clearly showed that only DNA enters the bacterial host and not the protein. DNA, therefore, is the infective part of virus and carries all the genetic information (Fig. 1).