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Pneumococcus Transformation Experiment
2
Studies on Chemical Nature of Substance Inducing Transformation of Pneumococcal Types
Vaishnavi Suren & Margaret Lee || 3B
Background
Background and Hypotheses
- 1928-1944: scientists believed proteins were the carriers of genetic information in organisms
- The Griffith Experiment proved cells have transferrable genetic material through testing strains of dead and living Streptococcus pneumoniae on rats
- The Avery-Macleod-McCarty Experiment set out to determine that DNA carries genetic information using genetic transformation of bacteria Streptococcus pneumoniae
- Both experiments use heat-killed cells to prove the composition and role of material in living ones
Hypotheses
- Material in dead bacterial cells can transform material in living cells
- The base of genetic material in cells is either of proteins or of nucelic acids.
Procedures
- Type III S Pneumococcus: Smooth strain; covers itself with polysaccharide capsule with antibodies that protects it from host's immune system. This is the virulent strain.
- Type II R Pneumococcus: Rough strain; no protective shield and is killed by host's immune system. This is the non-virulent strain.
Procedures
Griffith's Procedure
Four experimental groups:
1) Mice infected with R, nonvirulent strand
2) Mice infected with S, virulent strand
3) Mice infected with S strand that has been killed by heat
4) Mice infected with a mix of heat-killed S strand and live R strand
Griffith's Procedure
Avery-MacLeod-McCarty Procedure
1) Began with large cultures of heat-killed S cells
2) Purified the transforming principle by isolating it
3) Isolated it by washing away, separating out, or enzymatically destroying the other cellular components
4) R cells added to the purified substances to see if S cell production seen in Griffith's experiment occurs
Outcomes
The "Transforming Principle" proved to be DNA.
- No S-Cells appeared upon addition of R-Cells in the solution with DNase
- The mice were only killed when either live viral strands or mixed live R and dead S strands infected them
Outcomes and Data
"it is of special interest that in the example studied, highly purified and protein-free material consisting largely, if not exclusively, of desoxyribonucleic acid is capable of stimulating unencapsulated R variants of Pneumococcus Type II to produce a capsular polysaccharide identical in type specificity with that of the cells from which the inducing substance was isolated."
—Avery-MacLeod-McCarty Experiment Paper
Griffith Outcome
Griffith Outcome
Mice infected with either live virulent strands or a mix of live nonvirulent and dead virulent died. Griffith concluded that the R bacteria took up a "transforming principle" (what we now know as DNA) from the S bacteria.
Avery-MacLeod-McCarty Outcome
Avery-MacLeod-McCarty Outcome
The elemental composition of the purified transforming substance resembled the nitrogen and phosphorus levels of DNA.
Setting the Stage
The Avery-MacLeod-Mccarty Experiment strongly suggested that DNA was the Transforming Principle, allowing the Hershey-Chase experiment to prove this through observing the structures of DNA and proteins in cells after injecting them with these molecules.
Bibliography
Bibliography
Avery, Oswald T., et al. “Studies on the Chemical Nature of the Substance Inducing Transformation of Pneumococcal Types.” Journal of Experimental Medicine, Rockefeller University Press, jem.rupress.org/content/79/2/137.full.pdf+html.
“Classic Experiments: DNA as the Genetic Material.” Khan Academy, Khan Academy, www.khanacademy.org/science/biology/dna-as-the-genetic-material/dna-discovery-and-structure/a/classic-experiments-dna-as-the-genetic-material.
Cornell, Brent. “DNA Experiments .” BioNinja, ib.bioninja.com.au/higher-level/topic-7-nucleic-acids/71-dna-structure-and-replic/dna-experiments.html.