Griffith was trying to produce a vaccine for pneumonia by growing Pneumococcus pneumoniae (The genus has been changed to Streptococcus).
On the Petri dish, Griffith found out there were two forms. One was rough colonies which was smaller and slower growth (without capsule protected) and one was smooth colonies which was bigger (with capsule protected). He named them as "rough" and " smooth"
Griffith performed a series of test with "rough" and "smooth" on mice
Test of virulence of rough:
He injected the lyse rough bacteria (LR) into the mice. And after a period of time, the bacteria did not cause any diseases on the mice.
Test of virulence of smooth
Similarly, he injected the lyse smooth bacteria (LS) into the mice. However, the bacteria lived and rapidly divided which killed the mice.
Test of rough challenging with smooth
Initially, he injected LR into the mice, the mice survived as the first case, and then he challenged it with LS which finally killed the mice.
Test of Heat killed LS in order to confer immunity to smooth strain
He injected HKS (heat killed smooth) which allow the mice to live. Unfortunately, the mice died with another injection of LS.
Test of HKS + LR
He injected a mixture of HKS with LR which has killed the mice, He determined to observed the blood samples of these mice and he found out that LR is able to convert themselves into LS with some fragment of dead LS. He called it "Transferring Principles"
Avery, McCarty, and Macleod experiment (1944)
After Griffith left his experiment with no conclusion, Oswald Avery, Maclyn McCarty, Colin MacLeod used his experiment as a base to prove that DNA is a genetic material
They isolated a flask of Lyse smooth bacteria which was cell free extract and tested with six different tubes.
Tube 1: No enzymes, LR -> LS (positive result)
Tube 2: Lipase, LR -> LS (positive result)
Tube 3: Carbohydrase, LR -> LS (positive result)
Tube 4: Peptase, LR -> LS (positive result)
Tube 5: RNAase, LR -> LS (positive result)
Tube 6: DNAase, LR -> LS (negative result)
From this experiment, they concluded that DNA is a genetic material
Hershey and Chase experiment (1952)
Alfred Hershey and Martha Chase were the virologists studying the viruses that attack bacteria (or bacteriophage, "phage")
By using the fact that protein has sulfur and protein has phosphorus, they grew the virus and labelled some phage with radioactive sulfur and some with radioactive phosphorus.
First experiment, they added the bacteria and radio-labelled phosphorus into a Waring blender. After allowing the phages to inject DNA into the bacteria, a blender was turned on and got rid of most of the phages on the bacteria. Then centrifuging the mixture. Consequently, when analyzing the radioactivity of the supernatant and the pellet, the radioactive phosphorus only shown up in the pellet not the supernatant.
Second experiment, they performed the same procedures but this time, they used radio-labelled sulfur. As a result, the supernatant has the radioactive sulfur but not the pellet.
Irwin Chargaff experiment
By using paper chromatograph, he found out that the amount of purines = amount of pyrimidines or [Adenine]= [Thymine]. [Guanine]=[Cytosine], and [Adenine+ Guanine]= [Thymine+Cytosine]
Rosalind Franklin and Maurice Wilkins experiment
Rosalind Franklin was an expert of X-ray crystallographic and by the cooperation of different scientists, the structure of DNA was finally determined as a double helix structure with these following evidences:
1) A=T, G=C
2) 2x the mass of empirical formula
3) Nucleotide structure
4) Phosphodiester linkage
5) Built model
6) anti-parallel
7) helical
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