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Lecture 5 The chemical nature of the Gene
CAMPBELL BIOLOGY
Chapter 10:
The Molecular Biology of the Gene
Some elegant experiments that provided insights into fundamental concepts in genetics
(1) Evidence that Genes are located on Chromosomes
1902 – McClung – a particular chromosome (X) determines
sex in insects (XO = male; XX = female)
1903 – Sutton and Boveri – chromosomes behave just like
the ‘unit factors’ described by Mendel
1910 – Morgan – the “white” eye color gene of Drosophila
is located on the X-chromosome
- there are many other X-linked genes
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(2) But what are Genes made of?
1869 – Meischer
- he separated nuclei from cells
- examined their chemical composition
- extracted a phosphorus-rich polymer
called nuclein
- this was actually DNA
1912 – Feulgen
- developed stains for detection of proteins
- showed that the nucleus and chromosomes
contained proteins as well as DNA
(3) Problem: If chromosomes contain both DNA and proteins,
which one is the genetic material (genes)?
Arguments in favour of genes being made of proteins:
• 
Proteins are sufficiently complex to store genetic information
but DNA is not
• 
Proteins - contain 20 different subunits (amino acids) and
• 
There are many different types of proteins
• 
DNA - contains only 4 different subunits
• 
The complexity found in proteins was considered sufficient to
specify genetic complexity
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Arguments in favour of genes being made of DNA
•  All cells of a given species contain a constant amount of DNA
but the types and amounts of proteins differ in different cells
•  The amount of DNA doubles in every cell just before it divides
and an exactly equal amount is distributed to the two daughter cells
•  Gametes contain half the number of chromosomes and half the
amount of DNA
•  Therefore, although DNA is a very simple molecule, it behaves
in the way genetic material would be expected to behave
During the 1940’s the widely accepted conclusion was:
  Genes are likely to be made of protein
  DNA merely provides a structural framework in
chromosomes
Conclusive experimental evidence that genes are made of DNA
came from studies of: (a) bacterial transformation
and
(b) virus replication
Bacterial Transformation
1928  Griffith Streptococcus pneumoniae: two types
S strains - virulent:caused lethal pneumonia in mice
- can be isolated from blood of dead mice
- bacterium has a polysaccharide coat
- forms large, slimy colonies on agar plates
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R strains - non-virulent: doesn’t kill mice
- doesn’t make a polysaccharide coat
- grows as small, rough colonies on agar plates
Griffith’s experiments: inject mice with the different strain types
observe effect on mice and analyse their blood for bacteria
Strain injected
Type II R
Type III S
killed Type III S
killed Type III S + Type IIR
Effect
none dead
all dead
none dead
Blood analysis
no bacteria present
Type III S present
no bacteria present
all dead
Type III S present
Pretty amazing idea to decide to test this last category!
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Explanation: Bacterial Transformation
1.  The killed Type IIIS did not come back to life!
2.  Genetic material contained in the Type IIIS cells
has leaked out and entered the living Type IIR cells
3.  The genetic material has converted (or transformed)
cells of the Type IIR strain into Type III S cells
4.  The genetic material from the Type III S contains
the information for making the polysaccharide coat
required for virulence
5. Griffith didn’t realise this!
What is the transforming substance?
Griffith’s discovery of bacterial transformation provided a way of
identifying what chemical substance in the bacterium contained
the genetic information i.e. the genes themselves
1944 Avery, McLeod and McCarthy
Proved that the transforming substance = DNA
1.  Made a soluble cell extract from Type IIIS cell cultures
2.  Tested the cell extract for its ability to transform Type II R
into Type IIIS.
Before testing for transformation
3. Treated the extract with different enzymes i.e.
(a) one that destroys proteins = protease
or (b) one that destroys RNA = RNAase
or (c) one that destroys DNA = DNAase
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Transformation Results
Transformation
of TypeII R ?
A.  S extract
alone
Transforming
Substance
YES
?
B.  S extract + protease
YES
NOT PROTEIN
C.  S extract + RNAase
YES
NOT RNA
D.  S extract + DNAase
NO
must be DNA
Avery, McLeod and McCarthy
www.visionlearning.com/library/modules/mid149/Image/VLObject-3756-080922120939.jpg
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1953  A second experiment confirms that DNA = The Genetic
material
The Hershey-Chase experiment:
  Used a virus that infects and reproduces inside a bacterial cell
= phage
  The virus is made of 50% DNA and 50% protein
  Question: which substance contains the genetic information?
DNA contains phosphorus ( 32P) but no sulphur –
can “label” the DNA with radioactive 32P
Protein contains sulphur ( 35S) but no phosphorus
can “label” protein with radioactive S
Alfred Hershey and Martha Chase
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Experiment 1:
The protein coat of the phage is labelled with
The DNA is not labelled.
S
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The protein coat stays on the surface of the bacterium and can
be knocked off. The progeny phage contain no 35S
Experiment 2:
The DNA of the phage is labelled with
The protein is not labelled.
P
32
The phage injects its DNA into the bacterial cell. When the cells are
pelleted, the 32P is in the pellet. The progeny phage contain 32P
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