Griffith, Avery, & Hershey Experiments

Griffith experiment (1928)
Two strains of bacteria – smooth (killed mice) and rough (didn’t kill mice)
Smooth strain was heated (boiled) – protein died. Bacteria was dead. Didn’t kill mice.
Boiled smooth bacteria and then added it to the rough strain. Mouse was injected with live rough strain and dead smooth strain. Mouse died. Can grow smooth live bacteria from dead mouse’s heart.
We know protein is not molecule of inheritance – what experiment “proves”
Hypothesis – Protein is the molecule of inheritance. (Griffith disproved this hypothesis)
Conclusion – chemical from dead cell can transform living cell
Didn’t know DNA was molecule of inheritance

Avery-McLeod experiment (1944)
Offshoot of Griffith experiment
How is it different from Griffith experiment?
Different: treat bacteria with protease, RNase, DNase
After boiling smooth strain, they treat it in 3 different, separate ways: protease (breaks down protein), RNase (breaks down ribonucleic acid), and DNase (enzyme that breaks down DNA)
Each smooth strain which has been individually treated with different enzyme is put in test tube. These are exposed to live rough bacteria (rough bacteria mixed into each test tube)
Protease – no functional protein
RNase – no functional RNA
DNase – no functional DNA
Each of these enzyme/bacteria mixes are injected into separate mice.
Protease – mouse dies (protein not molecule of inheritance)
RNase – mouse dies (RNA not molecule of inheritance)
DNase – mouse lives (because of Dnase, DNA was killed. It was smooth DNA bacteria killing mice.)
This does NOT prove that DNA is molecule of inheritance. doesn't it mean that DNA is the molecule of inheritance because DNA is the transforming substance in the experiment? They just disproved a lot of things were NOT the molecule of inheritance.
This proves that lots of things are not molecule of inheritance. (protein, RNA, etc)
Used same kind of bacteria as Griffith experiment (smooth and rough)
Same: boiled smooth strain. Wanted to know if rough strain could be altered by smooth strain like Griffith experiment. Used mice.

“-Ase” – enzyme which breaks down the thing “-ase” is attached to (Ex tubulinase: enzyme which breaks down tubulin)


Hershey-Chase experiment (1952)
T4 phages and T2 very closely related
Used T2 phages and bacteria
T2 phages attach to proteins on outside of bacteria and inject their DNA into the bacteria. Bacteria replicates phage DNA. Bacteria explode in order to allow baby T2 phages out.
Scientists wanted to know what T2 phages were injecting into bacteria.
Hypothesis: Either DNA or protein might be the hereditary material that enters the bacteria from the T2 phage (virus).
Labeled DNA with radioactive phosphate. DNA is made of bases attached next to each other attached to sugar phosphate backbone. DNA has lots of phosphorus because of backbone.
Labeled protein with radioactive sulfur. Protein has lots of sulfur but little phosphorus
By labeling with radioactive elements, scientists could see where DNA and protein went.
After labeling with radioactive elements, phages are added to bacteria.
Bacteria/phage mix is put into blender separate the viruses from the bacteria. Stuff on outside (cell wall, etc) is heavier than the stuff on the inside. Outside stuff (virus) is supernatant fluid, and inside stuff (bacteria) is in pellet
Whatever is not molecule of inheritance will still be stuck to outside of bacteria – scientists not sure if that’s protein or DNA. Whatever is molecule of inheritance must be inside bacteria – scientists know this.
Most of the radioactive phosphorus is in pellet, and most of the radioactive sulfur is in the supernatant fluid. This means DNA is the molecule of inheritance.

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