The Genetic Material

Today, we all know that DNA is the genetic material that carries information from generation to generation. But, have you ever wondered how and when DNA was discovered? What experiments and observations led to the discovery of DNA? Let’s explore this fun journey of the discovery of the genetic material here.

Suggested Videos

 

Discovery of DNA

Scientists had narrowed down that the genetic material was on chromosomes in the nucleus of a cell. However, the exact molecule was discovered only much later. Let’s take a look at the series of experiments that scientists undertook that brought us closer to the discovery of DNA.

Frederick Griffith

While working with Streptococcus pneumoniae (the bacterium that causes pneumonia) in 1928, Frederick Griffith observed a miraculous transformation in this bacterium. When you grow this bacterium on a culture plate, some produce shiny colonies (denoted as ‘S’) and some produce rough colonies (denoted as ‘R’).

The S strain bacteria have a polysaccharide coat which gives rise to smooth, shiny colonies. The R strain lacks this coat and hence, it gives rough colonies. Also, the S strain is virulent and causes pneumonia; while the R strain is non-virulent. He performed the following experiment with these strains and saw different observations.

1. S strain → Inject into mice → Mice develop pneumonia and die.
2. R strain → Inject into mice → Mice live.
3. Heat-killed S strain → Inject into mice → Mice live. (Griffith found that heating kills the bacteria).
4. Heat-killed S strain + R strain → Inject into mice → Mice die.

• Observations – Not only did the mice injected with the heat-killed S strain + R strain die, but Griffith also recovered live S strain bacteria from these dead mice.
• Conclusions – He concluded that this was because the R strain had somehow been ‘transformed’ by the heat-killed S strain. This he argued was due to the transfer of a ‘transforming principle‘ from the S strain to the R strain, which made the R strain virulent. Although significant, his observations did not identify the biochemical nature of the transforming principle.

Oswald Avery, Colin MacLeod & Maclym McCarty

Avery, MacLeod, and McCarty, together set out to determine the biochemical nature of the ‘transforming principle’ identified by Griffith. These people purified DNA, RNA, and proteins from the heat-killed S strain and determined which macromolecule converted the R strain into the S strain.

• Experiment – They first treated the heat-killed S strain with proteases to break down proteins. Subsequently, they treated it with RNAses and then DNAses to break down RNA and DNA, respectively.
• Observations – Both protease and RNAse treatments did not affect the transformation of the R strain into the virulent one. Finally, treatment with DNAses inhibited the transformation of the R strain.
• Conclusions – They concluded that the genetic material is not protein or RNA, but it is DNA. However, this discovery was not accepted by all biologists.

Alfred Hershey & Martha Chase

Much earlier, scientists believed that the genetic material was protein. In 1952, Hershey & Chase were the ones to conclusively prove that DNA is the genetic material. They worked with bacteriophages – viruses that infect bacteria. A bacteriophage attaches and delivers its genetic material into a bacterial cell, where it generates more virus particles. Hersey & Chase used bacteriophages to experiment as follows:

• Labelling – Some viruses were grown on a medium containing radioactive phosphorus and some on a medium with radioactive sulfur.
• Viruses – grown on radioactive phosphorus have radioactive DNA but not protein since DNA contains phosphorus but protein does not. Contrarily, viruses grown on radioactive sulfur have radioactive protein but not DNA since DNA does not contain sulfur.
• Infection – The radioactive phages were then allowed to infect the bacteria – E. coli.
• Blending and Centrifugation – As the infection progressed, the viral coats were removed from the bacteria by blending. Then, centrifugation was used to separate the viral particles from the bacteria.

Observations – Bacteria infected with viruses that have radioactive DNA, were radioactive, while bacteria infected with viruses that have radioactive protein, were not radioactive.

Conclusions – This experiment conclusively showed that DNA is the genetic material transferred from virus to bacteria, and not protein.

Properties Of Genetic Material

For a molecule to act as the genetic material, it should have the following characteristics:

• Be capable of replication i.e. create its own replica.
• It should be stable, structurally and chemically.
• It must have the scope for slow changes (mutations) to evolve.
• Be expressed in the form of ‘Mendelian Characters’.

Although DNA is the genetic material in most organisms, in some viruses, RNA is the genetic material. In fact, according to studies, RNA was the first genetic material. But, since RNA is unstable, DNA evolved from RNA with chemical modifications, making it more stable and more fit to carry genetic information.

Solved Example For You

Q1: Hershey and Chase labelled viral DNA with which radioactive element in their experiment?

1. Potassium
2. Sulfur
3. Phosphorus
4. Calcium

Sol: The answer is ‘c’. They used radioactive phosphorus to label DNA since DNA contains phosphorus.