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Identifying DNA as the Hereditary Material

Identifying DNA as the Hereditary Material. Initially. People thought that proteins were the hereditary material. 1920’s Frederick Griffith . A microbiologist who studied the pathology of Streptococcus pneumoniae Used two strains of the bacterium

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Identifying DNA as the Hereditary Material

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  1. Identifying DNA as the Hereditary Material

  2. Initially... People thought that proteins were the hereditary material...

  3. 1920’s Frederick Griffith • A microbiologist who studied the pathology of Streptococcus pneumoniae • Used two strains of the bacterium • S-Strain – highly pathogenic but can be made non-pathogenic by heating it

  4. 1920’s Frederick Griffith • Used two strains of the bacterium • R-Strain – non-pathogenic What does ‘pathogenic’ mean?

  5. Pg 205

  6. Pg 205

  7. 1920’s Frederick Griffith • Discovered that mice died after being injected with a mixture of heat-killed S-strain and living R-strain bacteria

  8. 1920’s Frederick Griffith • He called this the transforming principle – because something from the heat-killed pathogenic bacteria must have transformed the living non-pathogenic bacteria to make them deadly

  9. 1944 Avery, MacLeod & McCarty • Oswald Avery, Colin MacLeod (both Canadian!) and Maclyn McCarty • Continued with Griffith’s work • Wanted to identify what IS the transforming principle

  10. 1944 Avery, MacLeod & McCarty • Prepared identical extracts of the heat-killed S-strain and added different enzymes to each extract • One enzyme destroyed RNA, one destroyed DNA and one destroyed proteins

  11. 1944 Avery, MacLeod & McCarty • Each extract/enzyme mixture was mixed with live R-strain cells RESULT? • The only extract that did not cause transformation was the one that was treated with the DNA-destroying enzyme

  12. 1944 Avery, MacLeod & McCarty RESULT? • Therefore the transforming principle is DNA

  13. 1952 Alfred Hershey & Martha Chase • Designed one of the most famous experiments in the history of genetics • Used bacteriophages (viruses that infect bacteria)

  14. 1952 Alfred Hershey & Martha Chase • Have an inner nucleic acid core and an outer protein coat, called a capsid Capsid DNA

  15. 1952 Alfred Hershey & Martha Chase • Their experiment was designed to find out which part of the virus – the DNA or the protein in the capsid – enters bacterial cells and directs the production of more viruses

  16. Used radioactive isotopes to trace each type of molecule • Radioactive sulfur (35-S) – since proteins contain sulfur and DNA does not • Radioactive phosphorus (32-P) – since DNA has phosphorus and proteins do not

  17. Result? Bacteria exposed to P-32 showed radioactivity, while those exposed to S-35 did not Provided conclusive evidence that viral DNA was transferred to bacterial cells and that viral DNA held the info needed for the viruses to reproduce

  18. Pg 207

  19. Determining the Chemical Composition & Structure of DNA

  20. 1869 Friedrich Miescher • Isolated the nuclei of WBC that he obtained from pus • Extracted a weakly acid substance containing nitrogen & phosphorus

  21. 1869 Friedrich Miescher • Called it nuclein (since it was found in the nucleus) • Renamed nucleic acid once the chemical composition was discovered

  22. Early 1900’s Phoebus Levene • Isolated two types of nucleic acids (RNA and DNA) • After many years of experiments, Levene proposed that DNA and RNA are made up of individual units that he called nucleotides

  23. Nucleotides have 3 main parts: • nitrogen-containing base • 5-carbon sugar • Phosphate group

  24. 4 types of nitrogenous bases – A, C, T and G Double ringed purines (A & G) Single ring pyrimidines (C & T)

  25. 1949 Erwin Chargaff • Studied the chemistry of nucleic acids • Came to two significant conclusions: • There is variation in the composition of nucleotides among different species

  26. 1949 Erwin Chargaff Came to two significant conclusions: • Nucleotides are present in characteristic proportions Pg 209

  27. Chargaff’s Rule • In DNA the % composition of adenine is equal to thymine • % composition of guanine is equal to cytosine

  28. 1951 Linus Pauling • Discovered that many proteins have helix-shaped structures

  29. 1953 Rosalind Franklin & Maurice Wilkins • used x-ray diffraction analysis of DNA to determine its structure • Franklin was able to conclude that DNA has a defined helical structure

  30. 1953 Rosalind Franklin & Maurice Wilkins • Franklin determined DNA has two repeating patterns at intervals of 0.34 nm and 3.4 nm

  31. 1953 Rosalind Franklin & Maurice Wilkins • Franklin observed how DNA reacted with water and concluded that nitrogenous bases were located on the inside of the helical structure, and the sugar-phosphate backbone was located on the outside

  32. 1953 James Watson & Francis Crick • Used the results, conclusions and conjectures of their peers and produced a model for DNA • In 1962 they were awarded a Nobel Prize (along with Wilkins)

  33. ...and Rosalind Franklin got...nothing.

  34. Pg 213

  35. Pg 208

  36. Pg 213

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