DNA Part I The History and Discovery of the Structure and Role of DNA

2 DNA –How its structure was discovered

3 Identifying DNA as a unique molecule Friedrich Miescher was a Swiss chemist and was the first to identify DNA as a unique molecule.

4 Staining of DNA reveals somatic cells have the same amount of DNA and half as much as gametes Robert Feulgen, a German chemist, found a staining technique that stains more or less strongly based in the amount of DNA present (called Feulgen stain). He found that all cells in an organism had the same amount of DNA except gametes, which had half the normal amount.

5 Staining of DNA reveals somatic cells have the same amount of DNA and half as much as gametes. Cells stained with Feulgen stain. It is the DNA and not the proteins that are visible under the microscope.

6 History of DNA Fred Griffith demonstrated that bacteria could be “transformed” from one strain to another by transferring genetic factor from one organism to another. He used two different strains of the same bacteria. One could cause pneumonia and the other could not.

7 The conclusion was that the bacteria had incorporated heredity factor from a source and in doing so expressed a new smooth trait. Griffith’s Experiment

8 Avery, MacLeod and McCarty examined the various molecules found in the S-strain Pneumococcus cells to prove that DNA was responsible for the transformation of the bacterial cells. Extending Griffith’s Experiment and Identifying DNA as the Transforming Factor

DNA is the Molecule of Heredity. When various isolated chemical components of the S-strain Pneumococcus cells was mixed the R-strain Pneumococcus cells, it was shown that the DNA from the S-strain cells, that caused trans- formation.

10 Experiment of Hershey and Chase Alfred Hershey and Martha Chase demonstrated the genetic material is DNA by using viruses that infect bacteria. These viruses only stay on the outside of the cell when infecting the cells. Also viruses are composed of protein and DNA. It is known that the virus injects its genetic material into the bacterium which had to DNA or proteins.

11 It demonstrated that DNA is the material that genes are made of and not protein. Experiment of Hershey and Chase

12 Chargaff's Rule -> A+G=C+T=50% Percentage of Various Nucleotides in Genome OrganismsATGC Humans Wheat Sarcina lutea T Based on the observations above, two rules can be deduced 1.A+G=C+T=50%. 2.The percentages of the nucleotide vary for different species Experiment Chargaff

13 Rosalind Franklin used x-ray crystallography to determine that DNA was double stranded, a helix, phosphates were on the outside and three distances, 2.0 nm,.34 nm, and 3.4 nm showed up in a pattern over and over again in the diffraction pattern. Work of Rosalind Franklin

14 Work of James Watson and Francis Crick 1.Determined that the sugar and phosphates were on the outside. 2. Determined that the nitrogenous bases were forming the rungs of the ladder. Based on the rules of Chargaff and the information from the work of Franklin, James Watson and Francis Crick, determined the structure of DNA by making models.

15 Based on the work Franklin’s x-ray crystallography, Watson and Crick found the bonding; two purines are too wide and would overlap. two pyrimidines are too far apart to form the hydrogen bonds. a purine and a pyrimidine however, are just right! Determining the Nitrogen Base Pairing

16 Chargaff felt there had been an injustice done when he did not receive the Nobel Prize in 1962 along with Watson, Crick and Wilkins. Wilkins’ contribution to the structure of DNA was to show James Watson the work of Rosalind Franklin without her permission. Franklin did not share the Nobel Prize as she passed away from ovarian cancer in 1958 and posthumous nominations are forbidden. Chargaff’s Snub