Download Pivotal Experiments

Pivotal Experiments
Frederick Griffith 1927






Worked with non-virulent bacteria
Identified two types of colonies that grew after streaking of agar plate.
The most prevalent colony type had a rough cell appearance
When this was delivered to rats via aerosol, that rats survived
The second type of colony that was observed had a smooth appearance.
When the rats were treated with this colony type they died.
Next:
 Took rough colonies and added heat attenuated smooth strain (non-virulent),
the rats then died after treatment.
 This suggested that there was a conversion of the strain taking place
 This turned out to be Transformation.
Avery 1946




Repeated Griffith expts
Again showed that smooth could covert rough to a virulent strain.
Isolated out the DNA and was able to prove that DNA was the material that was
being passed on.
Saw that the converted rough strain had DNA from the original rough strain as
well as the non-virulent smooth strain
The Experiment



First they treated the bacteria with centafugation, which eliminates large cellular pieces.
The result: bacteria still transformed
Added protease, which removes all proteins
The result: bacteria still transformed
Treated the bacteria with deoxyribonuclease, which eliminates all DNA
The result: no transformation in the bacteria
The trio concluded that DNA is the cause of transformation, where in this experiment virulence is
inherited.
Chargaff 1950



Isolated out DNA
Showed that A paired with T and that C paired with G
Therefore this allowed the idea of complimentary strands to be explored
Hershey/Chase 1952






Further confirmed that DNA was the genetic material
Radiolabeled DNA with an isotope
Used bacteriophage to infect target cell.
Observed that DNA from bacteria was transferred to the target cell
Ex HIV virus targeting of the CD4 T helper cells
Control labeling of the bacterial protein showed that it remained in the
bacteriophage.
Watson/Crick 1953

.
Came up with double helix structure for DNA after looking at X ray analysis
conducted by Rosalind Franklin
CODON: a sequence of 3
nucleotides in mRNA that selects for an Amino Acid.
" A genetic 'code word' containing three 'letters.' "
" More than one 'word' for each amino acid. "
RNA Codon
Amino Acid