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Introduction

Previously, we have learned about DNA, the packaging of DNA, and an overview of its structure. The basic chemical composition of deoxyribose nucleic acid is nucleotides. DNA is a polynucleotide molecule that is known as the genetic material. Among all the different structures of DNA, the double-helix model of DNA is a well-known structure. We have discussed all the above information in detail through the previous blog.

In this blog, we will go through the truth of DNA or, say, Genetic material. That is what and who decide that DNA is the genetic material. There must be some experiments, some validation methods which confirm DNA and RNA is the genetic material. I hope you all know what genetic material is, as we have discussed in an earlier blog.

There are many macromolecules present in the cell-like carbohydrates, proteins, lipids, DNA, RNA etc. Then, what do you think is the reason behind proteins and other molecules except DNA and RNA are not genetic material?

Even after the successful discovery of nuclein, DNA acts as genetic material is not yet confirmed. Like us, scientists have also arrived with the same questions. So, they have performed numerous experiments to find out the actual genetic material. The transforming principle is one of the experiments performed on pneumonia-causing bacteria Streptococcus pneumonia.

Transforming principle of the genetic material

It is a series of experiments performed on the bacterium causing pneumonia by Frederick Griffith. The physical form of changes in bacteria becomes a remarkable point for his research. He grows some pneumococcus bacteria in the culture plate. He found some smooth, shiny (S) colonies and some rough colonies (R). Smooth colonies show mucous (polysaccharide) coat on it hence possess smooth texture while R strain lacks that coat.

He started a series of experiments using these grown strains on mice. First, he injects S strain that is virulent into mice and infects him. As a result, he found mice dead due to the development of pneumonia. After that, he infects mice with R strain and found that mice are alive without any infection.

After this experiment slot in the second slot, he successfully manages to kill the bacteria through heating. Then he injects the heat-killed S strain into mice and found mice alive without infection. Hence, he injects a mixture of heat-killed S strain and live R strain into the same mice in the third slot and found mice died due to pneumonia infection.

Griffith's Experiment - an overview | ScienceDirect Topics — Images By: Google | Image Source: Science Direct

Griffith concludes that S strain had transformed something which converts non-virulent R strain into virulent one from all these series of experiments. That transformed material modifies the R strain so that it can produce that polysaccharide coat on itself. Transformed material is non-other than something that can cause changes on a genetic level; hence, possibly genetic material.

This information was not sufficient to describe the biochemical nature of genetic material. Hence for more clearance and confirmation, there is a need to study the nature of genetic material.

Study of Biochemical Nature of Genetic Material

Initially, Protein is referred to as a genetic material before the confirmation of accurate genetic material. Many scientists like Oswald Avery, Colin MacLeod and Maclyn McCarty continue finding the biochemical nature of transforming material in the Griffiths experiment.

Hence, they extract DNA, RNA, and Protein from the heat-killed S cells to determine which one is transformed to change the nature of R cells. This study was performed using enzymes, namely-

Protein digesting enzyme proteases, RNA digesting enzyme RNases and DNA digesting enzyme DNase. The study results that DNA is the only material that transformed the avirulent R strain into virulent S strain. Because in the experimental study, RNases and Proteases do not affect transformation.

Finally, the genetic material is DNA!

After all the experimental studies still there is no firm evidence for DNA as genetic material. To confirm the transforming principle for the DNA, Alfred Hershey and Martha Chase perform experiments with the help of bacteriophages.

Bacteriophages meaning “bacteria eater”, are the viruses that infect the bacteria. These are composed of nucleic acid molecules surrounded by the protein structure. Possess their genetic material that is DNA and RNA encapsulated in the protein structure. They are found in the soil, water and other places where bacterias can be spotted. It binds on bacteria, infect and replicate with each division and increase their number. Subsequently, bacteria are producing more viruses and harming themself.

Hershey and Chase use this quality of replication or duplication of bacteriophage. They want to find the material which transforms from bacteriophage to the bacterial cell and infect the bacteria, whether it is Protein or RNA or DNA, with the help of radioactive substances.

Hence, they grow viruses using two different mediums: radioactive phosphorous and the second containing radioactive Sulphur. Viruses grow in the presence of radioactive phosphorous contained radioactive DNA. However, it does not grow in the presence of radioactive Protein as DNA contains phosphorous.

In the same way, viruses grow on the radioactive Sulphur containing radioactive Protein and not radioactive DNA because DNA does not contain Sulphur.

1 - Do Scientists Always Agree? (w/ Andrew Woodman) — Curioscity: a Science Show — Image By: Google | Image Source: Curioscity

In the above experiments, both types of radioactive phages are allowed to infect the bacteria. After infection proceeds, the empty virus coat, Protein in nature, is removed by agitation and then centrifuge the mixture for separation.

In the resulting solution, it is found that the bacteria-infected by radioactive DNA containing viruses only. Moreover, this final step indicates that DNA is the genetic material and not the Protein.

Who Is Genetic and Who Is Non-Genetic?

What is first and best, DNA or RNA?

As after so many experimental studies, it is confirmed that DNA is the genetic material and not the Protein. In some viruses, RNA is also acting as genetic material. Then why DNA is prominent over RNA? What can be the difference between DNA and RNA? Answers for such questions you will get in the upcoming blogs till the time stay tune.

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