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Discovering the "Transforming Principle"

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by

Jenny Weir

on 28 April 2016

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Transcript of Discovering the "Transforming Principle"

End
1928- Griffith
1940's: Oswald Avery
Avery's experiment to determine the molecule of the transforming agent:
Are viruses the same as bacteria?
Do they also transfer
DNA
or do they transfer
protein
?
James Watson & Francis Crick
JAMES WATSON
from Indiana
intrigued by this but didn't know any chemistry
goes to Cambridge in England
teams with Physicist interested in Biology (Crick)
Rosalind Franklin & Wilkins
Early 1950's
British Scientist
used technique called
X-ray diffraction
X-ray diffraction- process used for structural analysis
"Photo 51"
when she was gone, Wilkins showed this picture to Watson and Crick
Chargaff's Rule
biochemist
professor at Columbia
Created Chargaff's Rules:
Complimentary base pairing
In human DNA, for example, the four bases are present in these percentages: A=30.9% and T=29.4%; G=19.9% and C=19.8%
Relative amounts of guanine, cytosine, adenine and thymine bases
varies from one species to another
.
More evidence that DNA rather than protein could be the genetic material

Discovering the "Transforming Principle"
Chapter 12
Spanish influenza: pandemic 1918; killed 20 million- 40 million people; weakened immune system
Pneumoccocus- he was trying to find vaccine for bacteria

S strain- causes pneumonia, has smooth edge, has capsule, virulent
R strain- Harmless, rough edge, lacks capsule, nonvirulent
His Experiment
Conclusion:
Transformation
: Heat killed S-strain transfers disease causing genetic material to R-strain;
one strain of bacteria had been changed permanently into another
What was the transforming agent in Griffith's experiment?
Cells missing protein, carbs, lipids- mice died.
Cells missing DNA- mice would have survived.
Result: DNA is responsible for transformation in bacteria.
DNA = hereditary factor that can transfer harmless R cells into harmful S cells
Reaction?
35S
32P
Conclusion
: DNA is also the hereditary molecule in viruses
DNA is a nucleic acid made of 2 long chains/strands of nucleotides

-Nucleotide has 3 parts
:
1) 5 carbon sugar = deoxyribose
2) phosphate group= Phosphorous bound to 4 Oxygen
3) Nitrogenous base = Nitrogen, Carbon, hydrogen.
4 possible nitrogenous bases in DNA
2 are purines: Adenine and Guanine (A&G)
2 are pyrimidines: Cytosine and Thymine (C&T)
Complementary Base Pairs:
Adenine pairs with Thymine
Cytosine pairs with Guanine
A=T
C=G
1'
2'
3'
4'
5'
POLYMERIZATION:
3'
5'
5'
3'
sugar
sugar
phosphate
phosphate
phosphate
Sugar-phosphate
Backbone
Kind of boring...

Very structural in nature...doesn't seem that functional

So nobody was that impressed when Avery said that the transforming principle was DNA
Vocabulary:
Bacteriophage- virus that infects bacteria;
literally means bacteria "eater"
1952: Martha Chase and Alfred Hershey
How do we know which one?
Separate them and test it?
Chemical marker...
Radio active label:
1. What is in DNA but not in Protein?
2. What is only in DNA?

Grow Virus ------> 32 P labeled virus
(label DNA)

Grow Virus ------->35 S labeled virus
(label Proteins)
Bacteria = more dense and comes down; virus = more light (stays in supernatant)
VIRUS
April 1953:
Structure of DNA published
Watch the TedTalk with James Watson:

https://www.ted.com/talks/james_watson_on_how_he_discovered_dna#t-14026
Full transcript