The Discovery of DNA

The DNA Revolution In 1928, Griffith discovered that a factor in heat-killed, disease causing bacteria can “ transform” harmless bacteria into ones that can cause disease In 1944, Avery determined that genes are composed of DNA In 1952, Hershey and Chase concluded that the genetic material of the virus was DNA, not protein

1. One strain of bacteria (harmless strain) had been changed into another (harmful strain), this is called transformation. 2. A chemical substance (gene) from one cell is able to genetically transform another cell Living S (smooth) Strain (harmful) Living R (Rough) Strain (harmless) Kill the harmful S strain bacteria by heating Mix dead harmful S strain cells with living harmless R strain cells Mouse Dies Mouse healthy Griffith Experiment

Avery and DNA Experiment: -Step 1: Made extract, or juice, from the heat killed bacteria -Step 2: Treat the extract with enzymes that destroys proteins, lipids, carbohydrates and DNA -Step 3: Mix each kind of extract with harmless bacteria and see if transformation occurs -Result: Transformation occurs with all extract EXCEPT the DNA-destroyed extract -Conclusion: DNA is the nucleic acid that stores and transmits the genetic information from one generation of an organism to the next

Hershey and Chase They studied viruses: Non-living particles smaller than a cell that can infect living organisms. Bacteriophages: “bacteria eater” -They are composed of a DNA or RNA core and a protein coat

Protein on the virus are radioactively labeled ( 35 S) Virus infect bacteria with genetic material No radioactivity enters cell DNA in the virus are radioactively labeled ( 32 P) Virus infect bacteria with genetic material Radioactivity enters cell Hershey and Chase Experiment The genetic material of the virus was DNA Conclusion:

DNA Genes are made of DNA 3 critical things that genes were known to do: 1.Genes had to carry information from one generation to the next 2.They had to put that information to work by determining the heritable characteristics of organisms 3.Genes had to be easily copied

The Structural Discovery of DNA In 1952, Rosalind Francklin studied the DNA molecule using a technique called x-ray diffraction In 1953, James Watson and Francis Crick developed the double helix model of the structure of DNA

The Structure of DNA DNA: _____________________________________ _ DNA are composed of: ___________________

Purines and Pyrimidines Purine: DNA base which has two rings in their structure Pyrimindines: DNA base which has one ring in their structure

Chargaff’s Rule Erwin Chargaff had discovered that the percentages of G and C bases are almost equal in any sample of DNA; the same thing is true for A and T Watson and Crick later use chargaff’s rule to develop the base pairing rule: -A always pair with T in a double helix through hydrogen bonding -C always pair with G in a double helix through hydrogen bonding

Watson and Crick’s Model Using the x-ray evidence from Rosalind Franklin, Watson and Crick concluded that: -DNA has a double helix structure, in which two strands were twisted around each other -The deoxyribose and phosphate forms the backbone of the double helix; whereas the nitrogenous base are in the center of the double helix

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