1. THE CHEMICAL NATURE OF THE GENE
MOLECULAR GENETICS
THE CHEMICAL NATURE OF THE GENE
© 2016 Paul Billiet ODWS

2. What does a gene do?
The gene 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
© 2016 Paul Billiet ODWS

3. 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
© 2016 Paul Billiet ODWS

4. Four classes of molecules which could form genes
Biological macromolecules
Elements
Building Blocks
Polysaccharides
(carbohydrates)
CHO
Monosaccharides
Lipids
(Fats, oils and waxes)
CHO
Fatty acids (and glycerol)
Polypeptides
(proteins)
CHONS
Amino acids
Polynucleotides
(Nucleic acids)
CHONP
Nucleotides
© 2016 Paul Billiet ODWS

5. Griffiths (1928) Tried to determine what genetic material was made of.
profiles.nlm.nih.gov/CC/A/A/B/N/_/ccaabn_.jpg
© 2016 Paul Billiet ODWS

6. Griffiths’ Experiment
Pneumococcus bacteria on mice
2 STRAINS
S-type
Smooth colonies
Virulent
R-type
Rough colonies
Avirulent
Innoculate into mice
Dead from pneumonia
Not killed
© 2016 Paul Billiet ODWS

7. Griffiths’ Experiment
Live R-type (harmless) +
Heat-killed S-type
CONTROL
Live R-type only
CONTROL
Heat-killed S-type only
Mice died from pneumonia
No mice died
No mice died
Further test: Cultured lung fluid
Live S-type found
© 2016 Paul Billiet ODWS

8. 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
© 2016 Paul Billiet ODWS

9. Avery, MacCleod & McCarthy (1944)
Tried purifying the transforming principle to change R-type Pneumococcus to S-type
© 2016 Paul Billiet ODWS

10. Results Conclusion 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 not inhibited by RNAase
It was inhibited by DNAase
Conclusion
The transforming principle is a DNA
© 2016 Paul Billiet ODWS

11. Experiment Live R-type +
DNA extracted and purified from S-type bacteria
Experiment
Mice died from pneumonia
Live S-type bacteria cultured from the lung fluid
These S-type bacteria remained virulent for generation after generation
© 2016 Paul Billiet ODWS

12. 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 Purer extracts of DNA were better at transforming the bacteria types.
© 2016 Paul Billiet ODWS

13. Hershey & Chase (1952)
To determine the nature of hereditary material used by viruses
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14. Virus
A virulant sub-microscopic agent which will pass through filters that normally stop bacteria.
Ebola virus
© 2016 Paul Billiet ODWS

15. Virus Luria (1955) “bits of heredity in search of chromosomes”
Lwoff (1966) “viruses are viruses because they are viruses”.
© 2016 Paul Billiet ODWS

16. Virus Three properties of all viruses Infectivity
Absence of biochemical apparatus (no metabolism). A virus is not alive
Possess only one type of nucleic acid (DNA or RNA).
© 2016 Paul Billiet ODWS

17. Types of virus
Positive RNA viruses: TMV & Enterovirus (polio) RNA used like mRNA directly by the host cell for translation.
Negative RNA viruses: Ebolavirus, Orthomyxoviridae (influenza) Viral RNA is transcribed into positive RNA before it is translated. The RNA polymerase that does this is supplied by the virus.
Retrovirus: HIV Inject RNA strand plus reverse transcriptase. A DNA copy is made and used for the expression of viral genes.
Bacteriophages: DNA viruses.
© 2016 Paul Billiet ODWS

18. Virus structure Composition: Protein and nucleic acid
Organisation : Protein coat covering a nucleic acid core
A virus does not posses a cellular structure
Crystallisable.
© 2016 Paul Billiet ODWS

19. PHAGE ATTACK!
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20. PHAGE ATTACK!
© 2016 Paul Billiet ODWS
T2 Phage particle

21. E. coli bacterium before infection
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22. 4 min
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23. 10 min
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24. 12 min
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25. 30 min
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26. Virus “life” cycle Lysis Packaging Infection Protein synthesis
Replication
© 2016 Paul Billiet ODWS

27. The Herschey & Chase Experiment
Proteins: CHONS labeled with 35S
Nucleic acids CHONP labeled with 32P
Phage particle
© 2016 Paul Billiet ODWS

28. Milk shakes at Cold Spring Harbour
The Waring Blender
© 2016 Paul Billiet ODWS

29. The Herschey & Chase Experiment
Two types of T2 phages prepared: 35S labeled (proteins labeled) 32P labeled (nucleic acids labeled)
Infect fresh E. coli host cells with each type
Centrifuge to remove the surplus phages
Agitate in the blender to remove phages coats from host cells
Centrifuge again and collect: (a) phage coats (b) bacterial cells.
© 2016 Paul Billiet ODWS

30. Destination of the radioactivity / %
Results
Centrifuge fraction
Destination of the radioactivity / %
32P labeled phages
35S labeled phages
Bacterial host cells 80 20
Phages
© 2016 Paul Billiet ODWS

31. Observation
Most of the 32P (i.e. DNA) is injected into the host but not much 35S (i.e. protein).
© 2016 Paul Billiet ODWS

32. Error margin
The 20% 35P in the phage coat fraction phages that had not been removed by the first centrifugation plus attached phages that had not yet injected their DNA
The 20% 35S in the bacterial cell fraction could be due to phage coats which had not been successfully knocked off by the blender.
© 2016 Paul Billiet ODWS

33. Going further
Of the 32P which got into the host cells, half of it was found in phages produced from infected cells
Less than 1% of the 35S appeared in the next generation viruses.
Phage infecting a bacterium
© 2016 Paul Billiet ODWS