DNA As Genetic Material – Griffith, Avery, Hershey and Chase Experiments [PDF]

Nucleic Acids

Nucleic acids, the building block
of genome it can be either DNA or RNA. The genome is the entire set of genetic
material in a haploid cell. There are 46 or 23 pair of chromosomes are present
in a cell. The genome resides mainly in the nucleus called as the nuclear
genome which consists of 3200 million base pairs and about 25000 genes and
genome present in mitochondria called as the mitochondrial genome that consist
of 16.5kbp and 37 genes (as per human genome project). The DNA is proven as the
genetic material by different experiments which are, Griffith Transformation
experiment, Avery’s experiment and Hershey-Chase experiment.

1. Griffith transformation experiment

No machine-readable author provided. Madprime assumed (based on copyright claims)., CC0, via Wikimedia Commons

It was done by Fredrick Griffith,
a microbiologist in the year of 1928. In his study he used two different
strains of bacterium called Streptococcus pneumoniae, which are R-strain
(II R) and S-strain (III S). The S-strain was virulent strain that possessing a
capsule protective layer because of the presence of S allele, the capsule
protects the strain from enzymatic attack and they cause the disease. The
R-strain do not possess any capsule because of the presence of R allele and
hence they are avirulent.

Steps

1. A mice infected with S-strain
dies with pneumonia – because S-strain was virulent

2. A mice infected with R-strain
was alive – because R- strain was avirulent

3. A mice infected with heat
killed S-strain the mice alive – because the virulency of S-strain lost by heat
treatment

4. A mice infected with heat
killed S-strain mixed with R-strain the mice die – because something was
transferred to the avirulent R-strain from the heat-killed S-strain, making the
R-strain virulent

Griffith termed the transferred
particle as transforming principle, because the particle transferred was
capable of transforming the avirulent R-strain to virulent form. Griffith only
mentioned about the transforming material but he failed to explain which is
that material either DNA/RNA/Protein.

Then comes the second experiment
by three, Oswald T. Avery, Colin M. MacLeod and Maclyn McCarty and called as
Avery’ Experiment that was a continuation of Griffith’s Experiment.

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2. Avery’s Experiment

They used the transforming
principle and treated it with three different Enzymes ribonucleases,
deoxyribonucleases and Proteases. The transforming principle was the mixture of
heat killed S-strain and avirulent R-strain.

Steps

1. The transforming principle was
treated with ribonucleases enzyme the mice die – the RNA is being degraded here
but there was no effect and the mice dies, so it is found that the RNA is not
the one which is being transferred

2. The transforming principle was
treated with proteases enzyme the mice die – the protein is being degraded here
but there was no effect and the mice dies, so it is found that the protein is
not the one which is being transferred

3. The transforming principle was
treated with deoxyribonucleases the mice alive – the DNA Is being degraded and
indicated that the DNA is being transferred and transform the avirulent
R-strain to virulent form

So, from this experiment they
confirmed the DNA as the genetic material, also they done some analytical
experiments to prove this which are,

A. Ultracentrifugation –
suggested that the transforming principle settled fast, hence it has high
molecular weight

B. Gel Electrophoresis – the
transforming principle showed high rate of mobility based on charge to mass
ratio i.e., more negative moves fast

C. UV Absorption
Spectrophotometry – transforming principle shown absorbance at 260nm (only
nucleic acids)

D. Elementary chemical Analysis –
determined the Nitrogen to Phosphorous Ratio, in which the transforming
principle shown more phosphorous

These all-analytical experiments
come to the conclusion that the DNA is proven as the genetic material.

After all these experiments were
proved DNA as genetic material two other scientist namely Alfred D. Hershey and
Martha Chase was not satisfied they believed that the protein should be the
genetic material and they done another experiment with bacteriophages.

3. Hershey and Chase
Transduction Experiment or Blender Experiment

​/wikipedia.org user Thomasione, GFDL, via Wikimedia Commons

They done the experiments in the
year of 1952 with a T₂ phage, a bacteriophage/virus that infect E.
coli. T₂, a bacterial virus that composed of a DNA core and a
protective shell made up of a number of different protein molecules.

• As the protein coat will contain Sulphur atoms
  it was labelled with radioactive isotope ³⁵S and the DNA core
  contains phosphorous hence it was labelled with radioactive isotope of ³²P
  by growing the bacteriophages in ³⁵S containing medium and ³²P
  containing medium respectively.
• These labelled bacteriophages were used to
  infect the E. coli.
• After the infection the phage particles will be
  released to the bacterium for its multiplication
• Viral particles and bacterial cells were
  separated using centrifugation, and the radioactivity measured in which the
  protein coat with ³⁵S were obtained in the supernatant and bacterial
  cell inside with the viral DNA with ³²P were obtained the pellet.
• This clearly indicate that the virus transfers
  its DNA for the multiplication hence it was proved the DNA is the genetic
  material.

Neethu Krishna

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Qualified CSIR - National Eligibility Test (NET), eligible for Assistant Professorship in any Indian university. Graduated from the University of Kerala with B.Sc. Botany and Biotechnology. Attained Post-Graduation in Biotechnology from the Kerala University of Fisheries and Ocean Science (KUFOS) with the fourth rank. Conducted various seminars and presentations. Experienced in ELISA, Blotting, and other Good Laboratory Practices. Attended a certificate course in Patent Analytics. Done 6 months of internship in ICMR - Vector Control Research Center, Puducherry. 3 years of tutoring experience.

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