The Discovery of DNA HHMI video - 18 minutes

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)

What information can be gained by Griffiths experiment?

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…

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

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

What information did the Hershey-Chase experiment provide?

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

Hershey & Chase’s Experiment

What about Chargaff?

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

Rosalind Franklin

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

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

Matthew Meselson & Franklin Stahl

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

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

DNA Replication