Several famous experiments were conducted to determine the role of DNA as genetic material. Those by Griffith, Avery, and Hershey and Chase were the most illuminating. Griffith observed the phenomenon of transformation of genetic material. Avery concluded that DNA was the genetic material that gets transmitted from one generation to the next. The Hershey-Chase experiment demonstrated that the “mendelian factors” controlling inheritance of genetic traits are DNA, not protein.
The genetic information of an individual is present in the nucleotide sequences of DNA and RNA, which determine the phenotype of an individual. A gene is a fundamental biological or hereditary unit that is transmitted from one generation to another.
To understand the phenomenon of inheritance, scientists conducted several experiments to study the chemical nature of genes. The total number of genes on a single chromosome is different in every organism. For example, E. coli bacteria contain more than 3000 genes on single chromosome, whereas the bacteriophage virus R17 consists of only three genes.
Griffith’s Transformation Experiment
In a famous series of experiments with Diplococcus pneumonia (bacterium that causes pneumonia), Frederick Griffith observed the phenomenon of transformation.
Transformation, one of several biological processes by which genetic material (DNA) is transferred between microbial cells.
The principle of the experiment was based on the virulence of one strain that causes pneumonia and death of the host (mice), whereas another virulent strain does not.
Griffith used two strains of Pneumococcus bacteria, type III-S and type II-R for his experiments. The major difference between these two types of Pneumococcus is that the III-S strain has a smooth polysaccharide coat, whereas II-R strain lacks this coat. The coat of the III-S strain makes it resistant to the immune system of mice thus causing pneumonia and death of the mice. On the other hand, the II-R strain gets destroyed by the immune system of the host.
Griffith injected strains of Diplococcus pneumoniae bacteria into mice in following four combinations shown below (Figure 8):
- Virulent strain S type III (causes pneumonia).
- Avirulent strain R type II (does not cause pneumonia).
- Heat-killed virulent strain S type III.
- Mixture of heat-killed virulent strain S type III + avirulent strain R type II.
Table showing results of the experiment conducted by Griffith
|S. No.||Strain Injected||Effect on Mice|
|1.||Avirulent R type II||Survived|
|2.||Virulent S type III||Dead|
|3.||Heat-killed S type III||Survived|
|4.||Heat-killed S type III + R type II||Dead|
Griffith established that the virulence of the S strain was destroyed by heating the bacteria. The chemical from the heat-killed bacteria transformed the avirulent bacteria into virulent, a change that caused death of the mice.
He was surprised to find that mice died when they were injected with a mixture of heat-killed S bacteria and living R bacteria, neither of which caused mice to die when they were injected alone.
Based on his observations, Griffith hypothesized that a chemical component from the virulent S cells transformed the R cells into the more virulent S form (Griffith, 1928).
Griffith was not able to identify the chemical nature of this “transforming principle” other than the fact the fact that it was able to survive heat treatment.
Later, various experiments performed by Avery, McLeod and McCarty and by Hershey and Chase established that DNA was the “transforming factor” in Griffiths experiments.
Avery and DNA
In 1944, Oswald Avery followed up on Griffith by new experiments. He isolated DNA from virulent strain (S type III) and applied the same in a cultured medium of avirulent strain (R type II) in following four combinations:
- DNA extract from heat-killed virulent strain S type III, which contains DNA, RNA, and proteins.
- DNA extract from heat-killed virulent strain S type III treated with the enzyme DNAase that destroys DNA (so that RNA and proteins are present, but DNA is absent).
- DNA extract from heat-killed virulent strain S type III treated with the enzyme RNAase that destroys RNA (so that DNA and Proteins are present, but RNA is absent).
- DNA extract from heat-killed virulent strain S type III treated with the enzyme protease that destroys proteins (so that DNA and RNA are present, but protein is absent).
Avery observed that transformation occurred only when DNA was present in the extract and there was no transformation when DNA was digested with DNAse enzyme. Based on his observations, he concluded that DNA was the genetic material that gets transmitted from one generation to the next and not the proteins.
Hershey and Chase experiment
In 1952, Alfred Hershey and Martha chase conducted experiments on T2 bacteriophage, which infects E.coli. The phage has two parts, head and tail. The head has an outer coat of protein and an inner core of DNA, whereas the tail contains only protein (Figure 9).
Phage infects bacteria by attaching its tail to the body of the bacteria. DNA then enters the bacterial cell and multiplies inside while the proteinaceous part remains outside the bacterial cell (Figure 10). After replication, phages come out of the bacterial cell and make the outer coating their own.
Hershey and Chase developed radioactive strains of T2 phage (Figure 11). In one sample, radioactive phosphorus [P32] was used to label phosphorus, which is a component of DNA. In another sample, radioactive Sulphur (S35) was used to label Sulphur, which is a component of the protein found in DNA.
These labelled DNA and protein T2 phages were used to infect two different samples of normal bacteria in culture medium. The newly multiplied viruses in the normal bacteria were examined for radioactivity.
Observation and Conclusion
When bacteriophages containing P32, could infect non-radioactive bacteria, all the infected cells became radioactive and was passed on to the next generation of bacteriophages. However, when the bacteria were infected with bacteriophages labeled with S35 and then the virus coats was removed, no radioactivity could be detected in the infected cells.
The above experiment provided strong evidence that DNA — and not protein– is the genetic material.