DNA AS A GENETIC MATERIAL

    The search for the hereditary material in the organisms began during the mid-nineteenth century.

The principle of inheritance was discovered by Mendel. Based on his investigation, Mendel

concluded that some factors‟ are transferred from one generation to another. Mendel's Law of

Inheritance was the basis for the researchers on genetic material. Keeping his conclusions in

mind, scientists who came after him, focused on chromosomes in search of genetic material.

Even though the chromosomal components were identified, the material which is responsible for

inheritance remained unanswered. It took a long time for the acceptance of DNA as the genetic

transformation. However, the hunt for the hereditary material continued and studies showed that

RNA replaces DNA in some viruses like Tobacco, Mosaic virus, Bacteriophage, etc.,

    Though both the nucleic acids can act as genetic material, DNA is much more preferred. DNA is

stable both chemically and structurally which make it well-built genetic material. RNA in

humans does not act as a genetic material but play various other roles such as an adapter,

enzyme, helps in protein synthesis, etc. RNA functions as a messenger for information to be

transferred. Following are the few key experiments that demonstrated the genetic material.

GRIFFITH’S TRANSFORMATION EXPERIMENT

In 1928, Fred Griffith performed an experiment that provided a foundation for the discovery that

DNA is the genetic material. He was studying the pathogenicity of Streptococcus pneumoniae.

This bacterium causes a form of pneumonia in humans. Griffith wasn't trying to identify the

genetic material, but rather, trying to develop a vaccine against pneumonia. In his experiments,

Griffith used two related strains of bacteria, known as R and S. The S-strain was virulent and the

R-strain was avirulent.

•  R strain: When grown in a petri dish, the R bacteria formed colonies, or clumps of related

bacteria, that had well-defined edges and a rough appearance (hence the abbreviation

"R"). The R bacteria were nonvirulent, meaning that they did not cause sickness when

injected into a mouse.

•  S strain: S bacteria formed colonies that were rounded and smooth (hence the

abbreviation "S"). The smooth appearance was due to a polysaccharide, or sugar-based,

coat produced by the bacteria. This coat protected the S bacteria from the mouse immune

system, making them virulent (capable of causing disease). Mice injected with live S

bacteria developed pneumonia and died. 

Griffith injected both S and R strains to mice. The one which was infected with the S strain

developed pneumonia and died while that infected with the R strain stayed alive. In the second

stage, Griffith heat-killed the S strain bacteria and injected into mice, but the mice stayed alive.

Then, he mixed the heat-killed S and live R strains. This mixture was injected into mice and the

mice died. In addition, he found living S strain bacteria in dead mice.

AVERY, MACLEOD, McCARTY EXPERIMENT: IDENTIFYING THE

TRANSFORMING PRINCIPLE

In the early 1940s, a team of scientists led by Avery, MacLeod, and McCarty, jointly begin 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. They used biochemical purification of

cellular fractions to determine that DNA and not RNA or protein was the transforming principle.

Avery, Macleod and McCarty isolated DNA from virulent strain (SIII) and applied the same in

culture medium of avirulent strain (RII) in following four combinations and recorded the results.

(i) DNA extract from heat killed virulent strain SIII which contains all the components i.e. DNA,

RNA and proteins.

(ii) DNA extract from heat killed virulent strain SIII treated with DNAase enzyme which digests

DNA, so there are RNA and proteins but no DNA.

(iii) DNA extract from heat killed virulent strain SIII treated with RNAase enzyme which digests

RNA, so there are DNA and Proteins but no RNA.

(iv) DNA extract from heat killed virulent strain SIII treated with protease enzyme which digests

proteins, so there are DNA and RNA but no protein.

From the above experiment they concluded that DNA was the genetic material and not the

proteins. Because the transformation occurred when DNA was present in the extract and there

was no transformation when DNA was digested with DNAase enzyme.

THE HERSHEY-CHASE EXPERIMENTS

    Much earlier, scientists believed that the genetic material was protein. It was known that some

viruses consisted exclusively of DNA and a protein coat and could transfer their genetic material

into hosts. In 1952, Hershey & Chase were the ones to convincingly 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.

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.

Viruses (T2 bacteriophage) were grown in one of two isotopic mediums in order to radioactively

label a specific viral component. Viruses grown in radioactive sulfur (35S) had radiolabelled

proteins Viruses grown in radioactive phosphorus (32P) had radiolabeled DNA. The viruses were

then allowed to infect a bacterium (E. coli). 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.

From the above experiment they observed that Bacteria infected with viruses that have

radioactive DNA, were radioactive, while bacteria infected with viruses that have radioactive

protein, were not radioactive. This experiment conclusively showed that DNA is the genetic

material transferred from virus to bacteria, and not protein.