Download DNA experiments exercise

Griffiths 1928
In 1928, Griffiths published the (then) astonishing finding that genetic information
from dead bacterial cells could somehow be transmitted to live cells.
He was working with two kinds of Streptococcus pneumoniae bacteria – live R forms, and
heat-killed S forms.
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The R form (Rough) are a non-virulent form of the bacteria.
The S form (Smooth) are virulent and cause fatal pneumonia in mice.
Look at the summary of Griffiths’ experiments above.
Experiment 4 seems to show that harmless Rough bacteria can be transformed into deadly Smooth
bacteria when they are mixed with the cell components of Smooth bacteria.
Explain why Griffiths needed to carry out experiments 1 to 3 in order to draw these conclusions from
Experiment 4.
Avery, MacLeod and McCarty 1944
Avery’s work, spread over 15 years, tried to identify the transforming substance.
To do this, they need to purify the substance and show that it, and it alone, could cause the
transformational change. They tested several possible “contender molecules” before finally reaching
the conclusion that it was DNA.
The purified substance was shown to have an effect at dilutions of 1 in 600 million. Although the
purified substance was almost pure DNA, they needed to eliminate other possible explanations.
Look at the summary of their experiments above.
What is the purpose of treating the purified cell components with:
a) protease
b) RNase
c) DNase
Explain how these results show that DNA is the transforming substance.
Chargaff 1951
Chargaff analysed the components of DNA from a wide variety of organisms. In
particular, he studied the ratio of bases to one another.
Source of DNA
Human
Sheep
Hen
Turtle
Salmon
Sea urchin
Locust
Wheat
Yeast
E. coli
Adenine
30.9
29.3
28.8
29.7
29.7
32.8
29.3
27.3
31.3
24.7
Guanine
19.9
21.4
20.5
22.0
20.8
17.7
20.5
22.7
18.7
26.0
Thymine
29.4
28.3
29.2
27.9
29.1
32.1
29.3
27.1
32.9
23.6
Cytosine
19.8
21.0
21.5
21.3
20.4
17.3
20.7
22.8
17.1
25.7
What do these data reveal about the ratios different bases?
Watson and Crick used this information as one of their key insights into the double helix structure of
DNA.
What other details of the structures of the bases helped confirm their insight?(think about your
model...)
When a double stranded DNA molecule is exposed to high temperature, the two strands separate
and the molecule loses its helical shape. We say the DNA has been denatured.
What other conditions might denature DNA?
Why do the two strands separate but the strands themselves remain intact?
Regions of the DNA that contain many A-T base pairs are the first to become denatured as the
temperature of a DNA solution is raised.
Suggest why this might be, using your knowledge of the DNA molecule.