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Lecture no. 3
Historical perspectivesEarly evidence , DNA is the
genetic material-II
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Outline
Lecture no. 3
• How we know DNA is the genetic material?
• Search for genetic material---is it composed of
protein or nucleic acid /DNA or RNA?
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Griffith’s Transformation Experiment
Avery’s Transformation Experiment
Hershey-Chase Bacteriophage Experiment
Tobacco Mosaic Virus (TMV) Experiment
Chargaff’s Rules
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3 Characteristics of
“The Genetic Material”:
Lecture no. 3
 Must be a stable form containing information
about cell form and function.
 Must replicate accurately.
 Capable of change to allow evolution..
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Lecture no. 3
• Until the 1940s, the case for protein
seemed more likely. Why?
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Search for genetic material
Lecture no. 3
Timeline of events:
• 1890: Weismann - substance in the cell nuclei
controls development.
• 1900: Chromosomes shown to contain hereditary
information, later shown to be composed of protein &
nucleic acids.
• 1928: Griffith’s Transformation Experiment.
• 1944: Avery’s Transformation Experiment.
• 1953: Hershey-Chase Bacteriophage Experiment.
• 1956: First demonstration that RNA is viral genetic
material.
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Lecture no. 3
1928
1944
Griffith’s
Transformation
Experiment
Avery’s
Transformation
Experiment
Toward The Real
Conclusion
1953
Hershey-Chase
Bacteriophage
Experiment
1956
Tobacco Mosaic
Virus (TMV)
Experiment
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Lecture no. 3
First Real BreakFrederick Griffith’s Transformation Experiment.
• Griffith worked with two strains of bacteria Streptococcus
pneumoniae, IIR(non virulent) and IIIS (virulent).
• His experiment was one of the first suggesting that bacteria
are capable of transferring genetic information through a
process known as transformation.
Lecture no. 3
Continue…
1928
 Griffith concluded that the type II-R
had been "transformed" into the lethal
III-S strain by a "transforming factor“.
 Griffith hypothesized that the
transforming factor was a “IIIS”
protein.
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Lecture no. 3
Griffith’s “transforming factor” was different
from proteins because:
i.
The heat treatment did not denature the
unidentified factor;
ii. The factor was able to transfer information
from one type of bacterial cell to another;
iii. The factor transformed the host cells by
giving them new properties.
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Lecture no. 3
Continue…
Avery’s Transformation Experiment
1944
When DNA was destroyed, the transforming activity was lost,
but when DNA was left intact, the transforming activity survived
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Continue…
Avery’s Transformation Experiment
Lecture no. 3
1944
When DNA was destroyed, the transforming activity was lost,
but when DNA was left intact, the transforming activity survived.
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Lecture no. 3
Continue…
Hershey-Chase Bacteriophage Experiment
1953
Because DNA and not protein entered the
cell, then DNA must be the heritable
material (i.e., responsible for the function
and reproduction of phage).
A.H. received Nobel Prize in 1969 for this
& other work
Lecture no. 3
 Where was radioactive sulfur found at the end of
experiment 1?
 Where was it found at the end of experiment 2?
 Based on the Hershey–Chase experiments, is it
reasonable to assume that Griffith’s “transforming
factor” was DNA, not protein? Why or why not? What
is the connection between the two experiments?
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RNA as Genetic Material
Lecture no. 3
- Gierer & Schramm (TMV) Experiment
1956
Gierer & Schramm 1956/Fraenkel-Conrat & Singer 1957
Used 2 viral strains to demonstrate RNA is the genetic material of TMV.
In conclusion
Lecture no. 3
DNA (not RNA) is transforming agent
DNA (not protein) is the genetic material
RNA (not protein) is genetic material of some
viruses
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Lecture no. 3
Chargaff’s Rules 1949-1953, evidence for DNA as
genetic material.
Digested many DNAs and subjected products to chromatographic separation
Results:
A = T, C = G
A + G = C + T (purine = pyrimidine)
A + T does not equal C + G
Members of a species similar but different species vary in AT/CG ratio
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Lecture no. 3
 What could these observations suggest?
 What are possible relationships between (A) and
(T), and between (C) and (G)?
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