Discovering DNA By early 20 th century DNA
Discovering DNA By early 20 th century DNA composed of deoxy, P, A, C, T, G, Proteins thought to be the key to the gen
1931 1 Griffith Discovers Transformation 2 Bacteriologist F. Griffith discovers a variation in pneumonia bacteria: 1 • ‘rough’ PM; not coated doesn’t kill mice 2 • smooth variant has a polysaccharide coated PM; (unique protein) kills mice
Genetic Transformation trial 3 trial 4 Griffith uses heat to kill smooth bacteria (heat destroys proteins) Heat-killed smooth & rough strains of bacteria mixed together… mice live unexpected! mice die!
Conclusions 1) info from smooth bacteria passed into non-virulent bacteria; genetic code is changed; transformation 1) Bacteria cultures (‘offspring’) successfully kill mice – the change is heritable between generations 2) info is not likely a protein, since it is expressed despite heating
Avery, Mc. Carty, and Mac. Leod conclude it’s DNA that is transformed Walk through animation Used same bacteria strains as Griffith: heat-killed smooth non-pathogenic rough no RNA no proteins no lipids no carbs + x 2. mouse dies in all cases 4. Only destroying DNA prevents transformation of pathogen info; Code is DNA no DNA 1. Mix bacteria strains, plus an enzyme that destroys one macromolecule type 3. mouse lives!
The Hershey-Chase Experiment : viral DNA (not the protein coat) carries the genetic info Attached radioactive labels to the DNA & protein coat of a bacteriophage Protein coat These viruses invades bacteria, injecting its DNA, & leaving it’s protein coat behind http: //highered. mcgrawhill. com/olcweb/cgi/pluginpop. cgi? it=swf: : 535: : /sites/dl/free/0072437316 /120076/bio 21. swf: : Hershey%20 and%20 Chase%20 Experiment DNA
The Race to Discover DNA Structure Key discoveries Linus Pauling 1940 s Discovered the alpha-helical structure of proteins.
Key discoveries Chargaff’s Rule : the amount of adenine always equals the amount of thymine; same with cytosine and guanine http: //www. hhmi. org/biointeractive/media/DNAi_chargaff_ratio-lg. mov
Key discoveries Wilkins lab Franklin’s x-ray image of DNA (looking down on the molecule) gave Watson & Crick the last piece of the structural puzzle Franklin’s photo published in same issue of Nature Franklin’s work 1951 Photo 51 http: //www. hhmi. org/biointeractive/media/DNAi_pauling_triple_helix-lg. mov
Watson & Crick Nobel Prize, 1962 25 year old postdoc! http: //www. hhmi. org/biointeractive/media/DNAi_watson_basepairing-lg. mov
Structure deduced DNA is a double-helix A sugar-phosphate backbone 4 nucleotides, in 2 pairs: Adenine + Thymine Guanine + Cytosine
- Slides: 11