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MOLECULAR GENETICS THE CHEMICAL NATURE OF THE GENE © 2007 Paul Billiet ODWSODWS
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 ODWSODWS
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 ODWSODWS
Four classes of molecules which could form genes NucleotidesCHONPPolynucleotides (Nucleic acids) Amino acidsCHONSPolypeptides (proteins) Fatty acids (and glycerol) CHOLipids (Fats, oils and waxes) MonosaccharidesCHOPolysaccharides (carbohydrates) Building BlocksElementsBiological macromolecules © 2007 Paul Billiet ODWSODWS
Griffiths (1928) Tried to determine what genetic material was made of. National Library of Medicine's Profiles in Science © 2007 Paul Billiet ODWSODWS
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 © 2007 Paul Billiet ODWSODWS
Griffiths’ Experiment Live S-type found Further test: Cultured lung fluid No mice died Mice died from pneumonia CONTROL Heat-killed S-type only CONTROL Live R-type only EXPERIMENT Live R-type (harmless) + Heat-killed S-type © 2007 Paul Billiet ODWSODWS
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 ODWSODWS
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 ODWSODWS
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 ODWSODWS
Experiment Mice died from pneumonia Live R-type + DNA extracted and purified from S-type bacteria These S-type bacteria remained virulent for generation after generation Live S-type bacteria cultured from the lung fluid © 2007 Paul Billiet ODWSODWS
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 ODWSODWS
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