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Frederick Griffith (1928)- Gave mice the bacterium that causes pneumonia (2 strains)
- Smooth strain killed the mice (DEADLY)- Rough strain did not kill the mice (HARMLESS)- Killed smooth strain with heat, did not kill the mice- Mixed the harmless strain and the heat-treated deadly strain- He found that the harmless rough strain of pneumonia,
TRANSFORMED into a deadly form.
- He discovered the TRANSFORMING principle, genetic material (DNA or Protein?)
Oswald Avery(1944)- Wanted to test if protein was the transforming factor
- Treated Griffith’s mixture of heat-treated deadly strain and live harmless strain with protein-destroying enzymes.
- The bacterial colonies grown from the mixture were still transformed
- Then treated the mixture with DNA destroying enzymes.- The bacterial colonies failed to transform.- Determine that the transforming factor is DNA by testing
Griffith’s experiment
Alfred Hershey and Martha Chase(1952)
- Conducted experiments using viruses to be sure DNA was the transforming factor, not protein.
- A virus is DNA wrapped in a protein. They reproduce by infecting a living cell with genetic materal.
- A virus that infects bacteria is a bacteriophage.- Concluded that the phage’s DNA entered the bacterial cell during
infection but the proteins did not.
Roseland Franklin and Maurice Wilkins (1950s
- Developed X-ray crystallography. Provides clues into shapes and dimensions of complex milecules.
- Basic shape of DNA is a helix.
Watson and Crick- Modeled DNA’s structure.- Using Franklin’s work, created a new model in which two strands of
nucleotides wound about each other, forming a double helix
• Chargaff’s rules state that in any species there is an equal number of A and T bases, and an equal number of G and C bases
Sugar–phosphate backbone
5’ end
Nitrogenous
bases
Thymine (T)
Adenine (A)
Cytosine (C)
Guanine (G)
DNA nucleotide
Sugar (deoxyribose)
3’ end
Phosphate
