2. MOLECULAR GENETICS THE CHEMICAL NATURE OF THE GENE

3. 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

4. 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

5. 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

6. Griffiths (1928) Tried to determine what genetic material was made of. National Library of Medicine's Profiles in Science

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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