1. The Discovery of DNA HHMI video - 18 minutes

2. Explain how each of the following contribute to DNA knowledge?
Griffith’s experiment (1928)
Avery, MacLeod, McCarty (1944)
Hershey-Chase (1952)
Chargaff (early 1950s)
Maurice Wilkins & Rosalind Franklin (early 1950s)
Watson and Crick (1953)
Matthew Meselson & Franklin Stahl (late 1950s)

3. What information can be gained by Griffiths experiment?

4. 1928 – Frederick Griffith Transformation
Change in genotype and phenotype because of assimilation of external DNA by a cell
Used disease-causing (pathogenic) and non-disease-causing (nonpathogenic) bacteria and mice
But what caused the change in phenotype?? He wasn’t sure…

5. What information is shown by the Avery, MacLeod, McCarty experiment

6. 1944 – Avery, MacLeod, McCarty
Discovered that DNA is the “transforming agent”
Their finding was not believed by the scientific community because proteins were believed to be better candidates for the genetic material

7. What information did the Hershey-Chase experiment provide?

8. 1952 – Alfred Hershey & Martha Chase
Is it protein or DNA that is the genetic material?
Used bacteriophages (viruses that infect bacteria) to show that since DNA enters the bacterial cells, but protein doesn’t, DNA must be the genetic material

9. Hershey & Chase’s Experiment

10. What about Chargaff?

12. Early 1950s – Erwin Chargaff
Analyzed the base composition of DNA
Set the stage for the base-pairing rules
# of adenines = # of thymines
# of cytosines = # of guanines
Known as Chargaff’s Rule

13. Rosalind Franklin

14. Early 1950s – Maurice Wilkins & Rosalind Franklin
Rosalind Franklin took X-ray crystallography diffraction photograph of DNA

15. 1953 – James Watson & Francis Crick
Constructed model of DNA as a double helix
Purine + pyrimidine for consistent width
C-G  3 hydrogen
bonds
A-T  2 hydrogen

16. Matthew Meselson & Franklin Stahl

18. Late 1950s – Matthew Meselson & Franklin Stahl
Semi-conservative replication of DNA
Each new molecule of DNA (after DNA replication) contains 1 old and 1 new strand
Replication Video 4 min

19. Enzymes can only add in the 5’ to 3’ direction (to the 3 prime end) Replication Video

20. DNA Replication