DNA The Genetic Material

1. DNA The Genetic Material

2. History of Genetic Science • Create a time chart in your notebook- use the book to fill it in-20 min!

3. Scientific History • The march to understanding that DNA is the genetic material • Mendel (1857) • T.H. Morgan (1908) • Frederick Griffith (1928) • Avery, McCarty & MacLeod (1944) • Erwin Chargaff (1947) • Hershey & Chase (1952) • Franklin (Early 1950s) • Watson & Crick (1953)

4. 1857 Mendel’s laws of heredity • Law of segregation • each allele segregates into separate gametes • Law of independent assortment • genes on separate chromosomes assort into gametes independently EXCEPTION • linked genes

5. 1908 | 1933 Chromosomes related to phenotype • T.H. Morgan • working with Drosophila • fruit flies • associated phenotype with specific chromosome • white-eyed male had specific X chromosome

6. 1908 | 1933 Genes are on chromosomes • Morgan’s conclusions • genes are on chromosomes • but is it the protein or the DNA of the chromosomes that are the genes? • initially proteins were thought to be genetic material… Why?

7. 1928 The “Transforming Principle” • Frederick Griffith • Streptococcus pneumonia bacteria • was working to find cure for pneumonia • harmless live bacteria (“rough”) mixed with heat-killed pathogenic bacteria (“smooth”) causes fatal disease in mice • a substance passed from dead bacteria to live bacteria to change their phenotype • “Transforming Principle”

8. The “Transforming Principle” mix heat-killed pathogenic & non-pathogenic bacteria live pathogenic strain of bacteria live non-pathogenic strain of bacteria heat-killed pathogenicbacteria A. B. D. C. mice die mice live mice live mice die Transformation=change in phenotype something in heat-killed bacteria could still transmit disease-causing properties

9. 1944 DNA is the “Transforming Principle” • Avery, McCarty & MacLeod • purified both DNA & proteins separately from Streptococcus pneumonia bacteria • which will transform non-pathogenic bacteria? • injected protein into bacteria • no effect • injected DNA into bacteria • transformed harmless bacteria into virulent bacteria mice die What’s the conclusion?

10. 1944 | Avery, McCarty & MacLeod • Conclusion • first experimental evidence that DNA was the genetic material Oswald Avery Maclyn McCarty Colin MacLeod

11. 1952 | 1969 Hershey Confirmation of DNA • Hershey & Chase • classic “blender” experiment • worked with bacteriophage • viruses that infect bacteria • grew phage viruses in 2 media, radioactively labeled with either • 35S in their protein coat • 32P in their DNA • infected bacteria with labeled phages Why useSulfurvs.Phosphorus?

12. Protein coat labeled with 35S DNA labeled with 32P Hershey & Chase T2 bacteriophages are labeled with radioactive isotopes S vs. P bacteriophages infect bacterial cells bacterial cells are agitated to remove viral protein coats Which radioactive marker is found inside the cell? Which molecule carries viral genetic info? 32P radioactivity foundin the bacterial cells 35S radioactivity found in the medium

14. Blender experiment • Radioactive phage & bacteria in blender • 35S phage • radioactive proteins stayed in supernatant • therefore viral protein did NOT enter bacteria • 32P phage • radioactive DNA stayed in pellet • therefore viral DNA did enter bacteria • Confirmed DNA is “transforming factor” Taaa-Daaa!

15. 1952 | 1969 Hershey Hershey & Chase Martha Chase Alfred Hershey

16. Let the race begin! • 1950s – scientific community racing to find 3-D structure of DNA • Major players • Erwin Chargaff • James Watson & Francis Crick • Linus Pauling • Maurice Wilkins & Rosalind Franklin

17. 1947 Chargaff • DNA composition: “Chargaff’s rules” • varies from species to species • all 4 bases not in equal quantity • bases present in characteristic ratio • humans: A = 30.9% T = 29.4% G = 19.9% C = 19.8% RulesA = T C = G That’s interesting!What do you notice?

18. Rosalind Franklin (1920-1958) • Wilkins & Franklin used X-ray crystallography to study DNA structure • Diffraction pattern used to deduce 3-D shape of molecules • DNA was helical in shape!

19. 1953 | 1962 Structure of DNA • Watson & Crick • developed double helix model of DNA • other leading scientists working on question: • Rosalind Franklin • Maurice Wilkins • Linus Pauling Wilkins Pauling Franklin

20. 1953 article in Nature Watson and Crick Watson Crick

21. DNA Structure Revealed • 1953 – the puzzle pieces are assembled • Watson put sugar-phosphate chain on the outside & nitrogen bases on the inside • Sugar-phosphate backbone like the side ropes of a rope ladder. • Pairs of nitrogen bases, one from each strand, form rungs • The ladder forms a twist every ten bases • Watson (age 25) & Crick (age 37) publish a 1 page paper in Nature • Franklin’s name was not mentioned • Hear from Watson

23. Learning Check • Summarize the evidence that DNA is the genetic material that allows traits to be passed on from one generation to the next

24. Scientific History • March to understanding that DNA is the genetic material • Gregor Mendel (1857) • Traits are carried from one organism to another • T.H. Morgan (1908) • genes are on chromosomes • Frederick Griffith (1928) • a transforming factor can change phenotype • Avery, McCarty & MacLeod (1944) • transforming factor is DNA • Erwin Chargaff (1947) • Chargaff rules: A = T, C = G • Hershey & Chase (1952) • confirmation that DNA is genetic material • Watson & Crick (1953) • determined double helix structure of DNA

25. Any Questions??