2 IDENTIFICATION OF DNA AS THE GENETIC MATERIAL The genetic material must meet 4 criteria1. Information:2. Transmission:3. Replication:4. Variation:9-3
3 1928 - Frederick Griffith’s Transformation Experiments Two strains of Streptococcus pneumoniaeS SmoothSecrete a polysaccharide capsuleProduce smooth colonies on solid mediavirulentR RoughUnable to secrete a capsuleProduce colonies with a rough appearanceavirulentTwo coat typesIIIIIFour possible phenotypesIIS or IIIS or IIR or IIIR
4 Griffith’s Experiments Figure 9.1Rare mutations convert strainsS into R strain & vice versaBut mutations do not change coat typesII into III or vice versa9-6
6 Griffith’s Conclusions Something from the dead type IIIS transformed type IIR into type IIISCalled this process transformationThe unknown substance was termed the transforming principle9-9
7 1944 - Avery, MacLeod & McCarty Identify the Genetic Material Griffith’s transforming principle was the genetic materialTransformation assay to identify actual biomoleculeMajor constituents - DNA, RNA, proteins, carbohydrates, & lipidsMade cell extracts from type IIIS cells containing each of these macromolecules9-10
8 Avery’s ExperimentsMixed each extract with type IIR cells to test for transformationOnly extract containing purified DNA transformed type IIR to type IIISVerify that DNA, not RNA or protein, is the genetic material9-11Figure 9.3
9 1952- Hershey & Chase Confirm DNA is Genetic Material Using Phage T2
11 Phage Attached to E. coli Cell Figure: 10-04aCaption:EM of E. coli during infection by phage T2.
12 The Hershey and Chase experiment: Used radioisotopes to distinguish DNA from proteins32P labels DNA specifically35S labels protein specificallyInfect non-radioactive E. coli with radioactively-labeled phagesRemove phage coats from cellsIs 32P or 35S inside bacteria?9-14
13 Hershey & Chase Experiment Figure: 10-05aCaption:Summary of the Hershey–Chase experiment demonstrating that DNA, and not protein, is responsible for directing the reproduction of phage T2 during the infection of E. coli.
14 Hershey & Chase Experiment Figure: 10-05bCaption:Summary of the Hershey–Chase experiment demonstrating that DNA, and not protein, is responsible for directing the reproduction of phage T2 during the infection of E. coli.