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3.2 Unit 2 The variety of living organisms
Experimental Evidence for DNA
Experiment 1:
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Performed by Frank Hershey and Martha Chase in 1952.
They were trying to prove that DNA rather than protein provided the genetic code.
Background:
 Bacteriophages are viruses that infect bacterial cells.
 The T2 bacteriophage infects Escherichia coli.
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The phage injects its DNA into the bacterium.
Consequently, the bacterium replicates the phage DNA, and synthesizes new
phage protein coats.
 New phage viruses are assembled which burst out of the bacterium.
Hershey and Chase set up 2 cultures of E.coli and phage.
Culture A contained radioactive phosphorus – 32P:
o This labeled the phage DNA.
Culture B contained radioactive sulphur – 35S:
o This labeled the protein coats.
New phage viruses were separated from each culture.
They were then separately reintroduced to further cultures of E.coli.
After a suitable length of time, the phage ‘heads’ were detached from the E.coli in a
blender then separated using a centrifuge.
The 2 fractions (phage heads and E.coli) were then tested for radioactivity:
Phage type
A - 32P
B - 35S
E.coli fraction
Radioactive
Non-radioactive
Phage head fraction
Non-radioactive
Radioactive
New phage offspring
Radioactive
Non-radioactive
What conclusions can you draw from this experiment?
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3.2 Unit 2 The variety of living organisms
Experimental Evidence for DNA
Experiment 2:
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In 1951, the American chemist Erwin Chargaff analysed DNA.
He used chromatography to separate the 4 bases in DNA from different species.
The quantities of each base were measured quantitatively:
Organism
Yeast
Locust
Human
TB bacterium
Quantity of each base / %
C
G
18
18
21
21
19
19
34
36
A
32
29
31
16
T
32
29
31
14
What conclusions can you draw from this experiment?
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EXPERIMENT 3
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DNA was extracted from a sea urchin and from a bacteriophage virus.
They analysed the composition of bases in their DNA.
Then, their DNA was heated to separate the DNA strands in the double helix.
They measure the heat required to separate 50% of the DNA.
Organism
A
Sea Urchin
32.8
Bacteriophage 9.9
C
% base composition
G
17.7
40.0
17.7
40.0
T
32.1
10.1
Separation
temperature
/ oC
71
86
What conclusions can you draw from this experiment?
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3.2 Unit 2 The variety of living organisms
Experimental Evidence for DNA
Experiment 4:
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In 1958, Meselson and Stahl grew E.coli bacteria in a medium containing amino acids
made with a heavy isotope of nitrogen – 15N.
The E.coli absorbed these amino acids and used them to synthesise nucleotides.
They incorporated these nucleotides in their DNA when it was replicated prior to cell
division.
Eventually, all of the DNA in the E.coli contained 15N.
This was demonstrated by extracting DNA from the E.coli and centrifuging it in a
density gradient medium.
Depending on the mass of DNA, it settles out at a particular point in the solution.
The E.coli were then transferred to a medium containing normal 14N.
Their DNA was extracted and analysed by the same method following each of the next
3 cell divisions.
1. Complete the table:
% DNA in sample
Generation
0
I
II
III
15N
14N
100
0
2. What conclusions can you draw from this experiment?
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