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DNA The Genetic Material

DNA The Genetic Material. Scientific History. The march to understanding that DNA is the genetic material T.H. Morgan (1908) Frederick Griffith (1928) Avery, McCarty & MacLeod (1944) Hershey & Chase (1952) Watson & Crick (1953) Meselson & Stahl (1958). Genes are on chromosomes.

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DNA The Genetic Material

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  1. DNA The Genetic Material

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

  3. Genes are on chromosomes • T.H. Morgan • working with Drosophila (fruit flies) • genes are on chromosomes • but is it the protein or the DNA of the chromosomes that are the genes? • through 1940 proteins were thought to be genetic material…

  4. 1928 The “Transforming Factor” • Frederick Griffith • Streptococcus pneumonia bacteria • was working to find cure for pneumonia • harmless live bacteria mixed with heat-killed infectious bacteria causes disease in mice • substance passed from dead bacteria to live bacteria = “Transforming Factor”

  5. The “Transforming Factor” 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? something in heat-killed bacteria could still transmit disease-causing properties

  6. 1944 DNA is the “Transforming Factor” • Avery, McCarty & MacLeod • purified both DNA & proteins 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 What’s the conclusion?

  7. 1944 Avery, McCarty & MacLeod Oswald Avery Colin MacLeod Maclyn McCarty

  8. 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 proteins • 32P in their DNA • infected bacteria with labeled phages Why useSulfurvs.Phosphorus?

  9. 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

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

  11. Hershey & Chase Martha Chase Alfred Hershey

  12. 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% That’s interesting!What do you notice?

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

  14. Watson and Crick Watson Crick

  15. Rosalind Franklin (1920-1958) X-ray Diffraction: 3-D structure of DNA

  16. Copying DNA • Replication of DNA • base pairing allows each strand to serve as a template for a new strand

  17. Can you designa nifty experimentto verify? Models of DNA Replication • Alternative models • so how is DNA copied? conservative semiconservative dispersive

  18. 1958 Semi-conservative replication • Meselson & Stahl • label “parent” nucleotides in DNA strands with heavy nitrogen =15N • label new nucleotides with lighter isotope = 14N parent replication Make predictions…

  19. 1958 Semi-conservative replication • Make predictions… • 15Nstrands replicated in 14N medium • 1st round of replication? • 2nd round? where should the bands be?

  20. Meselson & Stahl Matthew Meselson Franklin Stahl Franklin Stahl Matthew Meselson

  21. Flow of genetic information in a cell transcription translation DNA RNA protein replication

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