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Unraveling the Mystery of DNA: Key Discoveries and Structure Explanation

Explore the pivotal discoveries by Griffith, Avery, Hershey, and Chase that identified DNA as the genetic material, leading to the understanding of DNA structure like the double helix. Learn about nucleotides, Chargaff’s Rules, and the X-ray evidence by Franklin. Discover how Watson and Crick deciphered the DNA structure.

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Unraveling the Mystery of DNA: Key Discoveries and Structure Explanation

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  1. DNA Scientists Ch. 12-1 p. 286

  2. Griffith and DNA • Griffith and Transformation • Look at the results • How did this happen? • Transformation – one strain had been changed into another strain • Must be a gene that was transferred and caused the transformation

  3. Avery and DNA • Repeated Griffith’s work to figure out which molecule was important for transformation • What does this mean? • DNA stores and transmits the genetic information

  4. Hershey-Chase Experiment • Wanted to find out whether genes were made of DNA or proteins • Bacteriophages – virus that infects bacteria; “bacteria-eater” • Experiment • Took 2 viruses • Marked 1 for DNA, marked the other for protein • mixed them with bacteria • What do you think happened?

  5. Hershey-Chase Experiment • Only the DNA marker showed up, not the protein marker • What does this mean? • It means that genetic material is made of DNA, not protein

  6. Components and Structure of DNA • Gene requirements • Carry information from one generation to the next • Determine heritable characteristics of organisms • Had to be easily copied

  7. Components and Structure of DNA • DNA is made of nucleotides • Nucleotides have 3 parts • Sugar – deoxyribose • Phosphate group • Base – nitrogen containing • Purines – two rings • Adenine (A) • Guanine (G) • Pyrmidines – one ring • Thymine (T) • Cytosine (C)

  8. Components and Structure of DNA • DNA has a backbone • Sugar – deoxyribose • phosphate • Bases can be put in any order, like the letters of the alphabet • Each order means something different

  9. Chargaff’s Rules • The number of G is almost exactly equal to the number of C • The number of A is almost exactly equal to the number of T • This rule applied to all living organisms – flies, dogs, humans, trees

  10. X-ray Evidence and the Helix • Rosalind Franklin • Aimed X-rays at DNA samples and got an X-shaped picture • She didn’t quite know what it meant • Watson and Crick • Were trying to shape DNA • Saw Franklin’s picture and knew • IT WAS A DOUBLE HELIX

  11. Double Helix • Looks like a twisted ladder • What holds the two pieces together? • hydrogen bonds • Can only form between base pairs • A-T and C-G

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