The Discovery of the Genetic Material

Geneticists knew the hereditary factor that helped to pass on the necessary information from the parent to the offspring. The progeny show similar characteristics of the parents. Hence, it was necessary to know the reason or the factor behind passing on the characteristics from the parent to the offspring. The experimenters working on the hereditary material looked for three main factors. The first factor involved the stability of the information. Unless the information required for the growth, development, and reproduction is stable, it won’t get passed to the next generation. The second factor considered was the accuracy of the hereditary factor in replication. Unless it replicates accurately, the hereditary factor won’t be able to pass on the same information to the next generation. The third factor plays a crucial role in variation.

Fredriech Mischer’s discovery of nuclein:

Fredriech Mischer isolated the pus cells from the waste bandage. He studied these cells in detail and found the hereditary material in the nucleus of the cells. Nuclein or the material obtained from the nucleus revealed carbon, nitrogen, oxygen, hydrogen, and phosphorus. It was a biochemical test for detection. After forty years of Mischer’s research work, scientists came up with chromosome structure. It revealed nucleic acid and protein.

Griffith’s Transformation Experiment:

Griffith worked with the hereditary material in the year 1928. He used Streptococcus pneumonia for his experiments. It is a pathogenic bacteria leading to pneumonia disease. The pathogenic strains of this bacteria show the presence of sugary coat. It helps in spreading the disease and increasing the virulence of the bacterium. Griffith worked with two main strains such as the S strain and the R strain. The S strain forms smooth and shiny colonies. These strains are highly infectious. They have a well-defined polysaccharide coat. Griffith used two types of S strains such as IIS and IIIS. The R strain forms rough colonies. It is a relatively harmless strain. It lacks the polysaccharide coat. Hence, it is not a virulent strain. The S strain occasionally mutates to R. The IIS strains got mutated to IIR strains. The mice injected with the mutated strain survived. Next, the experimenter tried injecting the IIIS strain in the mice. The mice died. The reason behind the death of these mice included the infection due to virulent IIIS strain. Next, the experimenters used heat-killed IIIS strains. The mice survived the infection. The heat treatment killed the bacterial cells. Hence, the infection did not spread. Finally, the experimenter tried injecting a combination of heat-killed virulent IIIS strain with IIR strain. The mice died. The probable reason for the death of the mice included the interaction between the IIR and the IIIS bacterial strains. Hence, Griffith concluded the concept of transforming principle. Something got transferred from the dead cells to the live cells. Griffith’s transformation experiment paved the way for other experiments determining the main transforming principle. 

Image 1: Griffith’s experiment: It describes the experimental procedure carried out by Griffith. He first injected the mouse with rough strain (IIR) to check for their survival. The mouse survived. Next, he injected smooth strain IIS. The mouse did not survive. Third, he injected heat-killed smooth strain. The mouse again survived. Finally, he mixed both rough strain and heat-killed smooth strain and injected into the mouse. The mouse survived.

Avery’s experiment:

Avery and colleagues tried identifying the transforming principle. The experiment initiated in 1930 and worked till 1940’s. The experimenters worked on the same S. pneumoniae bacterium with virulent IIIS strains. They lysed the IIIS cells with a detergent and subjected to centrifugation. The separation of cellular components gives rise to the cell extract excluding the debris. The extract culture mixed with the culture of IIR got plated on a suitable medium. The IIIS colonies grew on the medium. The scientists knew that the transforming principle could be DNA, RNA, protein or the polysaccharide. Hence, they treated the cells with various enzymes. Due to the enzymatic treatment, the polysaccharides and the proteins got degraded. The experimenters again checked for the transforming principle using the IIIS strains. Hence, they found out that DNA or RNA could be the genetic material. To confirm which of the two could be the transforming principle, they treated the RNA with a nuclease such as RNase. Through the repeated experiments, the experimenters realized DNA to be the transforming principle. 

Image 2: Avery’s experiment: It describes two main steps such as treatment of the sample containing a mixture of DNA and RNA first with the RNase and then with the DNase enzymes. After treating the mixture with the RNase, and culturing, the researchers plated the mixture on a suitable medium and observed the growth. In this case, the colonies grew. Next, they treated the mixture with DNase and conducted the same procedure. This time, the colonies did not grow. 

Hershey Chase experiment:

Hershey and Chase studied T2 bacteriophage life cycle and correlated with DNA in the year 1953. It follows a lytic pathway. First, the bacteriophage infects the bacterial cell and injects its genome inside. Upon the entry of the phage genome, the bacteria stops synthesizing its DNA. The phage diverts the bacterial machinery for the replication of its own genome. The progeny phages get released from the bacterial cells. The bacterial cells get lysed. Hence, the bacteriophage uses the bacterial machinery to proliferate. Hershey and Chase tried studying the DNA through this experiment. They used two types of radioisotopes such as ³²P and ³⁵S. The experimenters mixed the phages with radioisotopes. Next, the radioactively labeled T2 progeny infected the E coli cells. The infection of the bacteria with phages having ³²P radioisotope gave rise to progeny having the same. The phages having ³⁵S radioisotope did not pass on the isotope to the progeny phages. The ³⁵S radioisotope involves radiolabelling of the proteins. From the above experiment, the researchers concluded the DNA to be the genetic material. 

Image 3: Hershey-Chase experiment: It describes the discovery of DNA as the genetic material using radioactive isotopes such as ³⁵S (the first step) and ³²P (the second step).

Discovery of the RNA as the genetic material:

An example of an RNA virus includes the tobacco mosaic virus. It produces lesions on the tobacco leaves. It consists of an RNA molecule and a protein coat. Both have a spiral configuration. The protein molecule surrounds the RNA and protects it from the degradation by nucleases. 

References:

[1] Advanced Biology - Page 396, Michael Kent, 2000 Preview
[2] Genetics 101 - Page 35, Michael Windelspecht, 2007 ‎Preview

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