Discovery of DNA as genetic material

1. DISCOVERY OF DNA AS
GENETIC MATERIAL
DR. ANU P. ABHIMANNUE
ASSISTANT PROFESSOR
DEPT. OF BIOTECHNOLOGY
ST.MARY’S COLLEGE, THRISSUR
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2. Blending Inheritance
• Theory postulates that essence of each and every body part was put
into the sperm and egg and that at conception a blending essences
occurred.
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3. Features of genetic material
• Complex enough to encode tens of thousands of different proteins
• High fidelity during replication to be passed down to future
generations
• Very stable thereby preventing high rate of randomization
• Subjected to mutations
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4. Confusion!!!
• During 1940's it was known that chromosomes contained both DNA and
small basic proteins called histones.
• Scientific world was confused between DNA & protein as the genetic
material. But most considered it’s protein.
• DNA is a polymer of only 4 different nucleotides. Proteins, however, are
composed of 20 different amino acids and so had a very satisfying degree
of complexity.
• 3 experiments had cleared this confusion
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5. Experiment I:Discovery of Transformation
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Frederick Griffith: (1877–1941) was a
British bacteriologist
He was working on Streptococcus pneumonia that causes
pneumonia in human & is lethal to mice

6. 2 strains of Streptococcus pneumonia
S STRAIN
 Virulent
 Enclosed in polysaccharide capsule
 Have smooth appearance
R STRAIN
 Mutant
 Non-virulent
 No capsule
 Rough appearance
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7. Result
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8. Conclusion
Some principle from heat killed S strain cells converted R strain into S
strain, thereby transforming them.
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9. Experiment II : Nature of Transforming Principle
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Oswald Avery: (October 21,
1877 – February 20, 1955),
Canadian-American physician
and medical researcher
Colin Munro MacLeod:
(January 28, 1909 – February
11, 1972), Canadian-
American geneticist
Maclyn McCarty: June 9,
1911 – January 2, 2005),
American geneticist

10. Protocol
1
• S strain in liquid medium was centrifuged
• The pellet cells were heat killed
2
• Cells were homogenized and recovered through filteration
• Treated with different enzymes – Dnase, Rnase, Protease
3
• Treated S strain was combined with R strain
• Observed for transformation
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12. Conclusion
• DNA could be the hereditary material leading to transformation!!!
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13. Experiment III : Blender experiment
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Alfred Hershey: December 4, 1908 – May 22, 1997)
was an American Nobel Prize – winning
bacteriologist and geneticist.
Martha Cowles Chase (November 30, 1927 – August
8, 2003), was an American geneticist

14. T2 bacteriophage
• Blender experiment was conducted
using T2 bacteriophage
• This was selected due to its simple
molecular constitution.
• The bacteriophage was radioactively
labelled with P32 (phosphorous is
found only in DNA) & S35 (sulfur is
found only in protein).
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15. Procedure
• P32 & S35 radioactively labelled bacteriophages were allowed to infect
E. coli cells separately
• After specific time of incubation, the empty phage carcasses known
as ghost cells were sheared by agitation in kitchen blender
• The radioactivity was measured in the 2 fractions and was observed
only in P32 infected cells and no radioactivity in S35 infection .
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17. Conclusion
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• Lack of 32P-labeled DNA remaining in the solution after infection suggested
transfer of phage DNA into the bacterial cell.
• All of the 35S in the protein coats remained outside the cell, showing it was
not incorporated into the cell, and that protein is not the genetic material.
• Hershey and Chase concluded that DNA, not protein, was the genetic
material.

18. Refernces
• https://en.wikipedia.org/wiki/Griffith%27s_experiment
• https://amm-dnaexperiment.weebly.com/
• https://en.wikipedia.org/wiki/Hershey%E2%80%93Chase_experiment
• Gerald Karp (2010). Cell and molecular biology: concepts and experiments (6th
ed.). John Wiley & sons. ISBN-13 978-0-470-48337-4.
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