1. St. Xavier’s College, Mahuadanr
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Nucleic Acid: Carries genetic material
Genetic Material-
Genetic material, also known as deoxyribonucleic acid (DNA) and ribonucleic acid
(RNA), plays a fundamental role in the composition of living organisms and responsible
for controlling hereditary characters.
Griffith’s Hershey & Chase
The process of transformation was first discovered by Frederick Griffith in 1928.
This was called as Griffith’s effect.
Transformation is the mode of exchange or transfer of genetic information
(recombination) from one strain of bacterium to another strain of bacterium
without involving any direct contact between them.
Griffith performed transformation experiments with two different strains of the
bacterium Diplococcus pneumoniae.
Diplococcus pneumonia is now named as Streptococcus Pneumoniae.
There are two types of strains i.e. Virulent and Avirulent strains.
Virulent strains are those which cause pneumonia in certain vertebrates (Such as
mice and human) and avirulent strains are those which do not cause pneumonia.
The difference in virulence is due to bacterium polysaccharide capsule is not
easily engulfed. Hence, they are able to cause pneumonia.
Non-capsulated bacteria are readily engulfed and destroyed by phagocytic cells.
On agar culture medium encapsulated strains forms smooth colonies and non-
encapsulated strains forms rough colonies.
They have different serotypes- i.e. IS, IIS & IIS or IR, IIR & IIIR.
2. St. Xavier’s College, Mahuadanr
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IIR &IIIS was used in the experiment serotype was identified by immunological
techniques.
Serotype Morphology Capsule Virulence
IIR Rough Absent Avirulent
IIIS Smooth Present Virulent
Experiment:
Live IIR-strain (Non-
virulent, non-
capsulated)
Injected into mice Survives
Live IIIS-strain
(virulent, capsulated)
Injected into mice Dies
Heat killed IIIS-strain Injected into mice Survives
Heat killed IIIS-strain
and live IIR-strain
Injected into mice Dies
Griffith injected different strains of bacteria in mice.
The IIIS strain killed the mice and IIR did not.
Heat killed IIIS strain did not cause pneumonia
Further heat killed IIIS and live IIR together injected to the mice, which
kills the mice.
Griffith concluded that heat killed IIIS is responsible for the conversion of
IIR in Virulent strain. This phenomenon is known as Transformation.
Genetic information is getting passed from dead IIIS to IIR cells by
transforming principle.
3. St. Xavier’s College, Mahuadanr
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Avery- Macleod-McCarty experiment
This experiment is also known as Transformation in Pneumococcus.
It is the direct evidence showing that the genetic material is DNA rather than
protein or RNA.
In 1944, Oswald Avery, Colin Macleod and Maclyn McCarty revisited Griffith’s
experiment and concluded that the transforming material was pure DNA not
protein or RNA.
They found that DNA extracted from virulent strain of the bacterium
Streptococcus pneumoniae, genetically transformed an avirulent strain of this
organism into a virulent form.
IIR-strain IIR-strain
+
IIIS DNA extract
IIR-strain
+
IIIS DNA extract
+
DNase
IIR-strain
+
IIIS DNA extract
+
RNase
IIR-strain
+
IIIS DNA extract
+
Protease
No transformation Transformation No transformation Transformation Transformation
IIR- strain being Non-virulent and non-capsulated shows no transformation.
When IIR-strain and IIIS DNA extract was injected into mice resultant
transformation took place.
Further IIR-strain, IIIS DNA extract and DNase was injected there was no
transformation because of DNA was destroyed by DNase.
Further experiment was continued with another combination i.e. IIR-strain, IIIS
DNA extract and RNase was injected. Resultant transformation took place. Here
RNA was destroyed by RNase.
4. St. Xavier’s College, Mahuadanr
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Another combination of IIR-strain, IIIS DNA extract with protease was injected
and showed transformation. Here protease destroyed the protein.
This experiment was concluded proving that DNA is responsible for genetic
information transfer and transformation. Because when DNase was used there was
no transformation and transformation occur when RNase and protease were used
as they contain DNA. Hence it was proved that genetic information carried by
DNA rather than protein or RNA.
5. St. Xavier’s College, Mahuadanr
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Hershey- Chase Experiment
Hershey-chase experiment was conducted in 1952, by Alfred Hershey &Martha
Chase using Phage and identified DNA to be the genetic material.
A phage is a virus that infects bacteria.
It consists of a protein coat that encloses the dsDNA.
Since phage consist only of nucleic acid surrounded by protein, it is easy to
determine whether the protein or the nucleic acid is the genetic material.
Experiment-
Radioactive isotopes of sulfur and phosphorus such as 35S and 32P were used.
T2- Bacteriophage and the bacterium Escherichia coli were taken for the
experiment.
The phages were labeled with radioactive isotopes 35
S and 32
P and infect the
bacterium growing in culture medium.
Further to identify the material infected into the cell by phages attached to the
bacterial wall.
6. St. Xavier’s College, Mahuadanr
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* E. coli was infected by 32
P labeled DNA phage. 32
P entered and next generation of
phages that burst from the infected bacterium results significant amount of 32
P label.
*E. coli was infected by 35
S labeled protein coat phage. There was no radioactive 35
S
in the next generation of phage.
*Experiment proved that genetic material in DNA not the protein.
Hershey shared the 1969 Nobel Prize in Physiology or Medicine for his
discoveries concerning the genetic structure of virus.
7. St. Xavier’s College, Mahuadanr
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Fraenkel-Conrat’s Experiment
The first experiment established by H. Fraenkel-conrat and B. Singer saying RNA
as the genetic material in RNA virus.
This experiment is known as reconstitution experiment.
They took two different strains of Tobacco Mosaic Virus (TMV), separated the
RNA from their protein coats and reconstituted hybrid viruses by mixing the
proteins of one strain with the RNA of the second strain and vice versa.
When hybrid virus was spread on tobacco leaves, the wound that developed
corresponded to the TMV from which the RNA had been obtained. Thus, it was
concluded that RNA serves as the genetic material in TMV.
