The History and the Discovery of DNA

Fredrick Griffith §Proved that traits were heritable, l traits could be passed from one generation to the next.

Griffith’s Experiment Mouse injected with Harmless Bacteria Mouse injected with Disease causing Bacteria Dead Mouse :( It’s Alive!

Griffith’s Experiment Injected with Harmless Bacteria, mixed with Dead disease Bacteria Injected with Dead disease causing bacteria Dead Mouse :( It’s Alive!

What did Griffith’s Experiment Show? §Traits are heritable l The killer bacteria passed the killing agent on to the harmless bacteria in Trial #4 l The Trait was passed physically, so it must be a molecule...

§2 possible molecules: Proteins/Amino Acids or Deoxyribonucleic Acid

Hershey and Chase §Set out to find whether Protein or DNA was the way traits are inherited §Known Info: l Protein had 20 different amino acids l DNA has 4 different nitrogen bases l What would you guess if you were a scientist at the time?

Hershey and Chase Experiment §Other known data: l Viruses are composed of only Protein and DNA l Viruses use other living cells to replicate themselves. l The part of the virus that enters the cell must be what contains the genetic info.

Parts of a Virus Capsule of the Virus (Made of Protein) DNA inside of the Virus

The Hershey and Chase Experiment §2 Cultures of Virus particles were “grown” in different liquids: l one “tagged” the DNA from the virus culture l another “tagged” the protein in the virus culture

The Hershey and Chase Experiment §Just after this, the solution is blended in a blender to shake the virus loose. § The 2 virus cultures are added to separate bacteria cultures § The virus injects the genetic info. into the bacteria

§The solution is spun in a centrifuge, which pulls the bacteria cells to the bottom of the tube, and the virus capsules remain in the solution §Results : The Hershey and Chase Experiment Virus Capsules Bacteria and Genetic info Radioactive DNA Radioactive Protein Virus Capsules Bacteria and Genetic info

What did this experiment show? DNA is the molecule that contains genetic info.

Determining the Structure of DNA §The Structure of DNA was determined by Francis Crick and James Watson

Determining the Structure of DNA §Known information about DNA: l It is composed of : a phosphate group a sugar group 4 different Nitrogen Bases –Adenine, Thymine, Guanine, and Cytosine

Determining the Structure of DNA Chargoff determined: Amount Adenine = Amount Thymine Amount Guanine = Amount Cytosine Nitrogen base pairing

Determining the Structure of DNA § Rosalind Franklin’s X-ray Crystallography of DNA showed the Double Helical structure of the molecule

Putting the Molecule together §Watson and Crick put together the DNA molecule using this info