Transcript File - Science with Masha

Griffith’s Mouse Experiment
Fredrick Griffith
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This experiment demonstrates
the power of genetic material.
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In his first experiment, Griffith wished to
determine the pathogenicity (disease –causing
capability) of his rough strain (Type IIR) of
Streptococcus bacteria
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To begin, Griffith injected cultures of
his rough strain into mice.
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Two weeks after injection, Griffith found that the
mice survived the introduction of the rough strain
into their systems.
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How would you interpret these results?
Select from the choices above.
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Smooth
In the second experiment, Griffith wished to determine
the pathogenicity (disease-causing capability) of his
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smooth (Type IIS) strain of Streptococcus bacteria.
To begin, Griffith injected living cultures of his
smooth strain into mice.
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Two weeks after injection, Griffith found that the mice
were killed as a result of the introduction of the
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smooth bacteria into their systems.
How would you interpret the results? Select from
the choices above.
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In his third experiment, Griffith wished to determine
whether the viability of the smooth strain was required for
pathogenicity (disease-causing capability). To do this he first
needed to kill these bacteria by boiling them for a short
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period of time.
Now that the smooth bacteria were dead, Griffith
could test whether or not they could cause disease in
that state.
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Griffith injected the heat killed bacteria into the
mouse.
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Two weeks after injection, Griffith found that the
mice survived the introduction of the heat-killed,
smooth bacteria.
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How would you interpret these results. Select
from the choices above.
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In his final experiment, Griffith wished to determine
whether the factor present in the living smooth bacteria
that causes disease could be transferred to nonpathogenic
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material.
Griffith first boiled pathogenic, smooth
bacteria, heat killing them .
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In the next step, Griffith needed to mix his heat-killed,
smooth bacteria, with living, rough bacteria.
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After mixing the two strains, Griffith injected a mouse
with the mixture. Neither the rough bacteria nor the
heat-killed smooth bacteria were capable of causing
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disease on their own.
The mixture was injected into mice, and the mice
were incubated for two weeks.
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After two weeks, Griffith found that the mice
died.
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How would you interpret these results? Select
from the choices above.
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The evidence for Griffith’s hypothesis came from an
investigation of the dead mice.
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When bacteria were recovered from the dead mice,
Griffith cultured them and found living, smooth
bacteria. Griffith reasoned that the only way for this
to have occurred was if living bacteria (rough, in this
case) were instructed to become smooth. Griffith
proposed the following explanation.
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Griffith proposed that when the smooth
bacterial culture was heat killed…
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…components present inside the smooth bacteria that
caused the bacteria to be pathogenic might have been
released into the media after death of the bacteria.
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Therefore, when nonpathogenic, rough bacteria
were introduced into the culture.
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…the cellular components from the killed, smooth
bacteria were able to enter the living, rough bacterial
cells.
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Once inside, the cellular components then transformed
the living rough bacteria cells into living, smooth cells.
Griffith therefore determined the cellular components
transforming factors. At that time, the exact molecule,
(or molecules) that make up the transforming factor
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were not known.
SUMMARY
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Hershey-Chase Bacteriophage Experiment - 1953
Bacteriophage = Virus that attacks
bacteria and replicates by
invading a living cell and using
the cell’s molecular machinery.
Structure of T2 phage
Bacteriophages
are composed of
DNA & protein
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Experiment
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Hershey-Chase Bacteriophage Experiment - 1953
1.
T2 bacteriophage is composed
of DNA and proteins:
2.
Set-up two replicates:
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Label DNA with 32P
Label Protein with 35S
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Infected E. coli bacteria with
two types of labeled T2
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32P
is discovered within the
bacteria and progeny phages,
whereas 35S is not found within
the bacteria but released with
phage ghosts.
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Conclusions about these early experiments:
Griffith 1928 & Avery 1944:
DNA (not RNA) is transforming agent.
Hershey-Chase 1953:
DNA (not protein) is the genetic material.
Alfred Hershey
Nobel Prize in Physiology or Medicine
1969
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Structure of DNA
James D. Watson/Francis H. Crick 1953 proposed the Double Helix
Model based on two sources of information:
1.
Base composition studies of Erwin Chargaff
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indicated double-stranded DNA consists of ~50% purines
(A,G) and ~50% pyrimidines (T, C)
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amount of A = amount of T and amount of G = amount of C
(Chargraff’s rules)
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%GC content varies from organism to organism
Examples:
%A
Homo sapiens
Zea mays
Drosophila
Aythya americana
31.0
25.6
27.3
25.8
%T
%G
%C
%GC
31.5
25.3
27.6
25.8
19.1
24.5
22.5
24.2
18.4
24.6
22.5
24.2
37.5
49.1
45.0
48.4
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Structure of DNA
James D. Watson/Francis H. Crick 1953 proposed the Double Helix
Model based on two sources of information:
2.
X-ray diffraction studies by Rosalind Franklin & Maurice Wilkins
Conclusion-DNA is a helical structure with
distinctive regularities, 0.34 nm & 3.4 nm.
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