MOLECULAR
GENETICS
THE CHEMICAL NATURE OF
THE GENE
© 2007 Paul Billiet ODWS
What does a gene do?


The must be able to hold information and decode
it (translate it) into an organism as it grows and
develops
It must be able to copy itself so that it can be
passed on to future generations
© 2007 Paul Billiet ODWS
What does a gene look like?


It must be a big molecule
to hold the large amount of information required
to build an organism
It must be a complex molecule
to provide the necessary variation to code the
instructions that control growth and development
© 2007 Paul Billiet ODWS
Four classes of molecules
which could form genes
Biological
macromolecules
Elements Building Blocks
Polysaccharides
(carbohydrates)
CHO
Monosaccharides
Lipids
(Fats, oils and waxes)
Polypeptides
(proteins)
CHO
CHONS
Fatty acids (and
glycerol)
Amino acids
CHONP
Nucleotides
Polynucleotides
(Nucleic acids)
© 2007 Paul Billiet ODWS
Griffiths (1928)
Tried to determine what
genetic material was
made of.
National Library of Medicine's Profiles in Science
© 2007 Paul Billiet ODWS
Griffiths’ Experiment
Pneumococcus bacteria on mice
2 STRAINS
S-type
Smooth colonies
Virulent
R-type
Rough colonies
Avirulent
Innoculate into mice
Innoculate into mice
Dead from pneumonia
Not killed
© 2007 Paul Billiet ODWS
Griffiths’ Experiment
EXPERIMENT
Live R-type
(harmless)
+
Heat-killed S-type
Mice died from
pneumonia
Further test:
Cultured lung fluid
Live S-type found
© 2007 Paul Billiet ODWS
CONTROL
Live R-type only
CONTROL
Heat-killed S-type
only
No mice died
No mice died
Conclusion


Transformation of R-type to S-type
Transformation was brought about by some heat
stable compound present in the dead S-type cells
Called the TRANSFORMING PRINCIPLE
© 2007 Paul Billiet ODWS
Avery, MacCleod & McCarthy
(1944)
Tried purifying the
transforming principle to
change R-type
Pneumococcus to S-type
National Library of Medicine's Profiles in
Science
© 2007 Paul Billiet ODWS
Results
The compound that had the most effect was:
 Colourless, viscous and heat stable
 It contains phosphorus
 It was not affected by trypsin (a protease) or amylase.
 It was inhibited by RNAase and DNAase
Conclusion
The transforming principle is a nucleic acid
© 2007 Paul Billiet ODWS
Live R-type
Experiment
+
DNA extracted and purified
from S-type bacteria
Mice died from
pneumonia
Live S-type bacteria cultured
from the lung fluid
© 2007 Paul Billiet ODWS
These S-type bacteria remained
virulent for generation after
generation
Conclusion
DNA is the transforming principle and it is hereditary
material
Criticism
The DNA was not totally pure
It was contaminated by a small amount of protein
This protein could be the real transforming principle
BUT
When Avery and his team prepared purer extracts of DNA
they became better at transforming the bacteria types
© 2007 Paul Billiet ODWS