Discovering DNA Timeline

  • Gregor Mendel

    Gregor Mendel
    Gregor Mendel studied inheritence. He observed variants of character, which he called traits in peas. He used different methods of pollinization to make generations of peas with different traits. His analysis of the peas led him to two laws of heredity: the law of segregation (describes dominant and recessive traits) and the law of independent assortment (Phenotypes are observable traits, while genotypes are the genetic make-up).
  • Morgan

    Morgan thought that the chromosomes were the location of Mendel's heritable factors. Through his fruit fly experiment, where he mated different varieties, and came to the conclusion that specific genes are carried on specific chromosomes. Genes located on sex Chromosomes exhibited unique inheritence patterns.
  • Federick Griffith

    Federick Griffith
    Griffith conducted an experiment where he injected two types of S. pneumoniae (a deadly and a non deadly variation) into mice. He conducted 3 trials injecting both substances into mice (seperatly and together). He did this experiment because he thought the killed deadly bacteria (killed when heated) passed on a characteristic to the non-deadly bacteria to make it virulent. He thought this characteristic was the inheritence molecule. He called this Transformation.
  • Oswald Avery

    Oswald Avery
    Avery continued with Griffith's experiment to see what the inheritence molecule was. In his experiment, he destroyed the lipids, RNA, carbohydrates and protiens from the virulent S. pneumoniae. However, after doing this, Transformation stil occured. When he destroyed the DNA in the virulent pneumonia, Transformation siezed. He concluded that DNA was the inheritence molecule.
  • Erwin Chargaff

    Erwin Chargaff
    Erwin thought that DNA was more than just repetitive tetra-nucleotide blocks. He isolated 4 nitrogenous bases from different organisms: Adenine, Guanine, Cytosine, and thymine. Purines have a double ring base, while pyrimidines have a single ring base. The amounts of Adenine to Thymine were proportional, and the amounts of Guanine to Cytosine were also proportional. This discovery was later known as Chargaff's rule.
  • Linus Pauling

    Linus Pauling
    Linus was one of the 1st scientiests to speculate about the structure of DNA. He proposed that the DNA was a triple helix based on the research he had compiled about the structures of amino acids and peptides. However, Rosalind Franklin's picture proved Pauling's proposal wrong. DNA was indeed a double helix.
  • Rosalind Franklin

    Rosalind Franklin
    Rosalind Franklin produced the pictures of DNA. She used two different fibers of DNA during the X-ray diffraction: one was more hydrated than another. From these pictures she deduced the basic dimensions of DNA strands. She knew that phosphates were on the outside of what probably was a double helix structure.
  • Hersey Chase

    Hersey Chase
    Hershey chase conducted an experiment using bacteriophge. He produced bacteriophage in a medium that labeled the protiens in the phage Sulfer-35 (radioactive). He also produced phage in a medium that labeled the DNA phospherous-32 (radioactive). When the phage infected the bacteria, The P-32 radioactivity was present in the next generation while the S-35 radioactivity was not. This proved that it is the DNA, not the protien coat that carries the genetic information.
  • Watson and Crick

    Watson and Crick
    Watson and Crick saw Rosalind Franklin's photos. This gave them enough information to put a model of DNA together. The model was a double helix with little rungs connecting the two strands. Watson and Crick realized that when the pyramidines (thymine and Cytosine) were paired together and the purines (Adenine and Guanine) were paired together, the DNA looked crooked. They paired A:T and G:C, which made the DNA look uniform. This went along with Chargaff's rule.