The discovery of DNA as the genetic material was one of the major achievements of science in the 20th century. This discovery made DNA, the chemical basis of heredity. It took almost 80 years for scientists to prove that DNA is the genetic material.
In 1869, Johann Friedrich Miescher, a young Swiss medical student, discovered an acidic substance that he isolated from pus cells obtained from bandages used to dress humans. He found it in the form of a mixture of compounds in the nucleus of the cell and named it “Nuclein”.
The nature of Nuclein was unusual as it contained large amounts of both nitrogen and phosphorous. At that time these two elements were coexisting only in certain types of fats. The discovery of Nuclein by Friedrich Meischer was quite early. Whereas it took a very long time to discover and prove that DNA is the genetic material.
By 1926, the quest to determine the mechanism for genetic material had reached the molecular level. In addition to this, different discoveries and findings further narrowed the search for the genetic material to the chromosome levels. But, the key genetic molecule was still missing from the research and findings.
Griffith’s Experiment: Transforming Principle
In 1928, Fredrick Griffith, an English microbiologist, transformed nonpathogenic forms of Streptococcus pneumoniae into pathogenic forms by changing their physical forms. He accomplished this transformation through the following steps:
- He injected mice with a mixture of heat-killed S strain (pathogenic) and live R strain (non-pathogenic) pneumococci. The mice died, and he recovered the live S strain of pneumococci from the blood of mice. That means heat-killed pathogenic strain transformed non-pathogenic live strain into pathogenic forms.
- But, when he injected live R strain (non-pathogenic) and heat-killed S strain separately, the mice lived. This is because none of them were pathogenic.
He concluded that heat-killed virulent S strain transferred some “transforming principle” to the non-virulent R strain, which enabled it to become virulent. This must be due to the transfer of genetic material.
In 1931, Richard Sia and Martin Dawson performed the same experiment in vitro, showing the mice played no role in the transformation process.
Evidence for DNA as the genetic material
For a very long time after the discovery of Nuclein, it was thought that proteins carry genetic information. Whereas, the nucleic acids were thought to have only structural functions. But somehow the search for the chemical identity of the transforming principle reached its climax in 1944.
In 1944, Oswald Avery, Colin Macleod, and Maclyn McCarty provided the first solid evidence of DNA as the genetic material. They were working on the transforming principle found in Griffith’s experiment.
They supported their evidence through the following observations:
- The transformation carried out by heat-killed S strain bacteria was inhibited, when the highly purified extract of that strain was subjected to the activity of Pancreatic Deoxyribonuclease (DNases). This is because DNAase digested DNA and hence no transformation occurred.
- They also found that Protein-digesting enzymes (proteases) and RNA-digesting enzymes (RNases) did not affect transformation. So they concluded that protein and RNA were not responsible for transformation.
Hence, they concluded that DNA is the genetic material that caused the transformation from heat-killed virulent bacteria to live non-virulent bacteria.
The second proof that DNA is genetic material was provided in 1952 by the experiments of Alfred Hershey and Martha Chase. They worked with bacteriophage T2, a virus that infects the bacterium Escherichia coli. The T2 phage consists of a DNA core that is surrounded by a protein coat.
They labelled the nucleic acid part of the T2 phage with radioactive phosphorus and protein coat with radioactive sulfur. After that, they infected E. coli cells with labelled T2 phage. They found out that the radioactive phosphorus (nucleic acid part) remained in the cells, whereas the radioactive sulfur (protein part) was largely lost.
This showed that proteins did not enter the E.coli from the T2 phage. Therefore, it was DNA that was transferred from the T2 phage to E. coli.
Also Read: Why Nature Preferred DNA over RNA?
So, now we know that the Hershey-Chase experiment unequivocally resolved the debate that DNA is genetic material. Thus, at last, the journey of genetic material from Nuclein to DNA came to an end and DNA is genetic material, became a fact. So that is all for now, meet you in my next article. Keep Reading, exploring, sharing your Knowledge, and above all, BE CURIOUS. 🙂
Also Read: 11 Amazing Facts about DNA You didn’t Know.