DNA The Genetic Material

Transcription

1 DNA The Genetic Material

2 Scientific History: The march to understanding that DNA is the genetic material u T.H. Morgan (1908) u Frederick Griffith (1928) u Avery, McCarty & MacLeod (1944) u Erwin Chargaff (1947) u Hershey & Chase (1952) u Watson & Crick (Franklin) (1953) u Meselson & Stahl (1958)

3 Chromosomes related to phenotype: T.H. Morgan u working with Drosophila fruit flies u associated phenotype with specific chromosome white-eyed male had specific X chromosome

4 Genes are on chromosomes: Morgan s conclusions u genes are on chromosomes u but is it the protein or the DNA of the chromosomes that are the genes? initially proteins were thought to be genetic material Why? What s so impressive about proteins?!

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

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

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

8 Avery, McCarty & MacLeod: 1944??!! Conclusion u first experimental evidence that DNA was the genetic material Oswald Avery Maclyn McCarty Colin MacLeod

9 Confirmation of DNA: Hershey & Chase Why use Sulfur vs. Phosphorus? u classic blender experiment u worked with bacteriophage viruses that infect bacteria u grew phage viruses in 2 media, radioactively labeled with either 35 S in their proteins 32 P in their DNA u infected bacteria with labeled phages Hershey

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

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12 Blender experiment: Radioactive phage & bacteria in blender u 35 S phage radioactive proteins stayed in supernatant therefore viral protein did NOT enter bacteria u 32 P phage radioactive DNA stayed in pellet therefore viral DNA did enter bacteria u Confirmed DNA is transforming factor Oh goodie, I like smoothies

13 Hershey & Chase: Hershey Martha Chase Alfred Hershey

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

15 Structure of DNA: Watson & Crick u developed double helix model of DNA other leading scientists working on question: wrosalind Franklin w Maurice Wilkins w Linus Pauling Franklin Wilkins Pauling

16 Watson and Crick: 1953 article in Nature Watson Crick

17 Rosalind Franklin ( ):

18 But how is DNA copied? Replication of DNA u base pairing suggests that it will allow each side to serve as a template for a new strand It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material. Watson & Crick

19 Models of DNA Replication: Alternative models u become experimental predictions Can you design a nifty experiment to verify? conservative semiconservative dispersive P 1 2

20 Semiconservative replication: 1958 Meselson & Stahl u label parent nucleotides in DNA strands with heavy nitrogen = 15 N u label new nucleotides with lighter isotope = 14 N The Most Beautiful Experiment in Biology parent replication 15 N/ 15 N 15 N parent strands

21 Experiment: 1 Bacteria cultured 2 in medium with 15 N (heavy isotope) Bacteria transferred to medium with 14 N (lighter isotope) Results: Most impressive! 3 DNA sample centrifuged after first replication 4 DNA sample centrifuged after second replication Less dense More dense

22 Conclusion: Predictions: First replication Second replication Conservative model Semiconservative model Dispersive model Why didn t I think of this?

23 Predictions: 1st round of replication 814 N/ 14 N 15 N/ 15 N 15 N/ 14 N 15 N/ 14 N P 1 2nd round of replication 15 N/ 15 N conservative 14 N/ 14 N 15 N/ 15 N semiconservative 414 N/ 14 N 15 N/ 14 N dispersive N/ 14 N 2 15 N parent strands conservative semiconservative dispersive

24 Meselson & Stahl: Matthew Meselson Franklin Stahl Franklin Stahl Matthew Meselson

25 Scientific History: March to understanding that DNA is the genetic material u T.H. Morgan (1908) genes are on chromosomes u Frederick Griffith (1928) a transforming factor can change phenotype u Avery, McCarty & MacLeod (1944) transforming factor is DNA u Erwin Chargaff (1947) Chargaff rules: A = T, C = G u Hershey & Chase (1952) confirmation that DNA is genetic material u Watson & Crick (Franklin) (1953) determined double helix structure of DNA u Meselson & Stahl (1958) semi-conservative replication

26 The Central Dogma : Flow of genetic information in a cell transcription translation DNA RNA protein replication

27 The continuity of life is based on heritable information in the form of DNA, and structure and function are correlated at all levels of biological organization. Describe how the structure of DNA is correlated with its roles as the molecular basis of inheritance.