Download Molecular Biology BCH 361

Lecture no.2
Introduction -2
BCH 361/ Section: xxxxx
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Lecture Outline
Lecture no.2
 Terminologies.
 Early experiments that led to the discover of DNA
as the genetic material.
 cellular components.
 Next time lecture.
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3 Characteristics of
“The Genetic Material”:
Lecture no.2
 Must be a stable form containing information
about cell form and function.
 Must replicate accurately.
 Must be able to change/evolve.
 Until 1944 it was not known which component of
chromosomes was the genetic material.
 Until 1953 it was not known how DNA could
encode genetic information.
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Early Studies
Lecture no.2
 Beginning with the earliest observations
concerning heredity, genetic material was
assumed to exist.
 Until the 1940s proteins were considered by
geneticists to be the best candidates:
 Very abundant in cells and did nifty things.
 Nucleic acids were similar, and just a couple of
nucleotides connected to each other…
Phoebus Levene
proposed a
tetranucleotide
for !!structure
DNA
Lecture no.2
 He though that a DNA molecule contained only four units, each unit contain
phosphate-sugar-base -in order- linked together in a repeated manner, i.e. a
tetranucleotide.
 Furthermore, he considered such a simple sequence could not allow DNA any role
in coding for anything.
 This was later to be proved wrong by Chargaff.
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So…
Lecture no.2
 It was widely thought that DNA was organized into
repeating "tetranucleotides" in a way that could
not carry genetic information.
 Proteins, on the other hand, had 20 different
amino acids and could have lots of variation.
 Most geneticists focused on “transmission
genetics” and passively accepted proteins as being
the likely genetic material
Lecture no.2
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.2
First Real BreakFrederick Griffith’s Transformation Experiment.
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.2
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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Lecture no.2
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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Lecture no.2
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
RNA as Genetic Material
Lecture no.2
- 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.2
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.2
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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