Give an experimental evidence to demonstrate that DNA is a genetic material.

Griffith’s Transformation Experiment (1928)

Frederick Griffith, while working with Streptococcus pneumoniae, observed a phenomenon called transformation. He used two strains of the bacteria: a virulent (S) strain with a polysaccharide capsule, causing pneumonia, and a non-virulent (R) strain lacking the capsule. His experiment involved the following:

• Injection of S strain: Mice died.
• Injection of R strain: Mice lived.
• Injection of heat-killed S strain: Mice lived.
• Injection of a mixture of heat-killed S strain and live R strain: Mice died, and live S strain bacteria were recovered.

Griffith concluded that some “transforming principle” from the heat-killed S strain had converted the R strain into the virulent S strain. However, he didn’t identify the nature of this principle.

Avery-MacLeod-McCarty Experiment (1944)

Building on Griffith’s work, Oswald Avery, Colin MacLeod, and Maclyn McCarty aimed to identify the “transforming principle.” They systematically isolated different components from heat-killed S strain bacteria and tested their ability to transform R strain bacteria.

Their experimental setup involved three sets of experiments:

• Treatment with protease (enzyme that degrades proteins): Transformation occurred.
• Treatment with RNase (enzyme that degrades RNA): Transformation occurred.
• Treatment with DNase (enzyme that degrades DNA): Transformation did not occur.

This experiment demonstrated that DNA, and not protein or RNA, was the transforming principle responsible for the genetic change. This was a crucial step in establishing DNA as the genetic material, although some scientists remained skeptical.

Hershey-Chase Experiment (1952)

Alfred Hershey and Martha Chase provided the most convincing evidence that DNA, not protein, is the genetic material. They used bacteriophages (viruses that infect bacteria) to demonstrate this. Bacteriophages consist of only DNA and protein.

Their experiment utilized radioactive isotopes:

• ³²P (radioactive phosphorus): Used to label DNA, as phosphorus is present in DNA but not in protein.
• ³⁵S (radioactive sulfur): Used to label protein, as sulfur is present in protein but not in DNA.

The experiment proceeded as follows:

• Bacteriophages were grown in media containing ³²P and ³⁵S to label their DNA and protein, respectively.
• The labeled phages were allowed to infect E. coli bacteria.
• After infection, the phage particles were removed by agitation, and the bacterial cells were analyzed.

Results:

• The bacterial cells contained most of the ³²P (DNA) inside.
• The bacterial cells contained very little ³⁵S (protein).

This conclusively showed that DNA, not protein, enters the bacterial cells during infection and is therefore the genetic material. This experiment effectively resolved the debate and solidified DNA’s role as the carrier of genetic information.

Further Evidence

Later, Chargaff’s rules (1950) regarding the base composition of DNA (A=T and G=C) and the X-ray diffraction studies by Rosalind Franklin and Maurice Wilkins (1952) provided further insights into the structure of DNA, ultimately leading to the discovery of the double helix structure by Watson and Crick (1953).