3.2 Unit 2 The variety of living organisms
Answers - Experimental Evidence for DNA
Experiment 1:
 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.







 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?
No 32P remained outside the cell which indicates that it was incorporated into DNA in the
bacterial cell. All of the 35S in the protein coats remained outside the cell, showing it was
not incorporated into the cell, and that protein is not the genetic material. A protein coat is
formed around the phage, but the bacterial DNA is what was used to produce new phages
inside a bacteria. Therefore the new phages from the bacteria infected with 32P phages,
also had 32P incorporated. The new phages from the bacteria infected with 35S phages,
had no 35S incorporated, because this was not available inside the bacterial cell.
3.2 Unit 2 The variety of living organisms
Experimental Evidence for DNA
Experiment 2:
 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?
In most organisms, A is in the same proportions as T, and C is in the same proportions as
G. This suggests in the DNA molecule there may be some connection between A and T
and also between C and G. The relative proportions of A with T compared to C with G are
different in different species.
EXPERIMENT 3




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?
A higher temperature was required to separate DNA when the composition of C and G is
higher. This indicates that C and G are bonded together more strongly than A and T. If
we presume that it is known that bases pair using hydrogen bonds, this suggests that
there are more hydrogen bonds between C and G than between A and T.
3.2 Unit 2 The variety of living organisms
Experimental Evidence for DNA
Experiment 4:
 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
50
25
12.5
0
50
75
87.5
2. What conclusions can you draw from this experiment?
DNA replication is semi conservative. Every time DNA replicates to produce two new
molecules, 50% of each of the new molecules consists of new nucleotides, but 50% of the
original molecule’s nucleotides are retained. This suggests that DNA consists of two
strands, which are separated during the replication process. 2 new molecules are created
by using the 2 separate strands from the original molecule as template on which new
nucleotides are assembled, thus producing 2 new DNA molecules.