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DNA: HOW IS WAS DISCOVERED AS THE GENETIC MATERIAL

DNA: HOW IS WAS DISCOVERED AS THE GENETIC MATERIAL. DNA, the substance of inheritance Is the most celebrated molecule of our time Hereditary information Is encoded in the chemical language of DNA and reproduced in all the cells of your body It is the DNA program

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DNA: HOW IS WAS DISCOVERED AS THE GENETIC MATERIAL

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  1. DNA: HOW IS WAS DISCOVERED AS THE GENETIC MATERIAL

  2. DNA, the substance of inheritance • Is the most celebrated molecule of our time • Hereditary information • Is encoded in the chemical language of DNA and reproduced in all the cells of your body • It is the DNA program • That directs the development of many different types of traits

  3. Concept 16.1: DNA is the genetic material • Early in the 20th century • The identification of the molecules of inheritance loomed as a major challenge to biologists WE ARE GOING TO GO THROUGH THE EXPERIMENTS THAT DISCOVERED DNA Was first worked out by studying bacteria and the viruses that infect them

  4. For each experiment answer the following • What are they trying to find out? • What did they do to try to answer their question • What did they conclude?

  5. Frederick Griffith Experiment

  6. 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

  7. Griffith called the phenomenon transformation • Now defined as a change in genotype and phenotype due to the assimilation of external DNA by a cell

  8. AVERY EXPERTMENT

  9. AVERY EXPERTMENT

  10. Alfred Hershey and Martha Chase: Experiment

  11. The Hershey and Chase experiment

  12. Hershey experiment summary • 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!

  13. Sugar-phosphate backbone Nitrogenous bases 5 end CH3 O– 5 O H CH2 O P O O 1 4 N O– N H H H H H O 2 3 H Thymine (T) O H H CH2 O O P N O N H O– H N H H H N N H H Adenine (A) H H O H N CH2 O O P H O O– N H N H H H O H Cytosine (C) O 5 H CH2 O P N O O O 1 4 O– H N H Phosphate H H N 2 H 3 DNA nucleotide N H OH N H Sugar (deoxyribose) 3 end H Figure 16.5 Guanine (G) Additional Evidence That DNA Is the Genetic Materia • Prior to the 1950s, it was already known that DNA • Is a polymer of nucleotides, each consisting of three components: a nitrogenous base, a sugar, and a phosphate group

  14. Chargaff 1947 • Erwin Chargaff analyzed the base composition of DNA from a number of different organisms • “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%

  15. Building a Structural Model of DNA: Scientific Inquiry • Once most biologists were convinced that DNA was the genetic material • The challenge was to determine how the structure of DNA could account for its role in inheritance

  16. Franklin’s X-ray diffraction Photograph of DNA (b) (a) Rosalind Franklin Figure 16.6 a, b • Maurice Wilkins and Rosalind Franklin • Were using a technique called X-ray crystallography to study molecular structure • Rosalind Franklin • Produced a picture of the DNA molecule using this technique

  17. G C A T T A 1 nm C G 3.4 nm C G A T G C T A T A A T T A G C 0.34 nm A T Figure 16.7a, c (a) Key features of DNA structure (c) Space-filling model • Watson and Crick deduced that DNA was a double helix • Through observations of the X-ray crystallographic images of DNA

  18. Franklin had concluded that DNA • Was composed of two antiparallel sugar-phosphate backbones, with the nitrogenous bases paired in the molecule’s interior • The nitrogenous bases • Are paired in specific combinations: adenine with thymine, and cytosine with guanine

  19. 5 end O OH Hydrogen bond P 3 end –O O OH O A T O CH2 O O P O –O O– O P O H2C O O G C O O CH2 O P O O O –O O– O– O– O P P P O O O H2C O O O O C G O O CH2 O P –O O H2C A T O O CH2 OH 3 end (b) Partial chemical structure 5 end Figure 16.7b WHAT DOES IT MEAN TO BE ANTIPARALLEL

  20. Watson and Crick reasoned that there must be additional specificity of pairing • Dictated by the structure of the bases • Each base pair forms a different number of hydrogen bonds • Adenine and thymine form two bonds, cytosine and guanine form three bonds

  21. H N O H CH3 N N N N H Sugar N N O Sugar Adenine (A) Thymine (T) H O N H N N N H N Sugar N N N O H Sugar H Figure 16.8 Cytosine (C) Guanine (G)

  22. Make the complementary strand

  23. SUMMARY OF THE STRUCTURE OF DNA

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