History of Genetics Flashcards
1865: Mendel
Gregor Mendel
–> Two laws of inheritance are identified through Mendel’s pea plant experiments
–> Identified “heritable factor” transmitted between generations
1910: Morgan
Thomas Hunt (T.H.) Morgan
–> Published study showing chromosomes were the carriers of Mendel’s “heritable factors” (genes)
–> It was uncertain whether these heritable factors on the chromosomes were DNA or protein
The case for protein as the heritable factor:
1) Proteins were more studied:
Seemed to have more diversity in the amino acids that was thought to allow for more combination = more complexity = more likely to be our genetic makeup
2) Nucleic acids seemed too uniform to account for the multitude of specific inherited traits
1928: Griffith
Frederick Griffith’s Streptococcus Pneumoniae Experiment
–> Founder of the “transformation principle”
Streptococcus Pneumoniae Experiment:
What is the difference between the strains?
Streptococcus Pneumoniae: Has 2 strains
1) R-Strain (Rough) = NON-pathogenic
–> Appearance: Smaller, no capsule
2) S-Strain (Smooth) = PATHOGENIC
–> Appearance: Bigger, HAS capsule
S-Strain Capsule Function
A polysaccharide coating that masks the bacteria from the immune system (hides it)
Genetic difference between R and S strains:
S-Strain = HAS the variant of the gene that encodes for capsule production
R-Strain = DOES NOT HAVE the variant of the gene that encodes for capsule production
Streptococcus Pneumoniae Experiment:
METHODS
1) Injected mice with different types of streptococcus pneumoniae strains
2) Analyzed whether mice died or not
3) Blood sampling to look at the bacteria strains
Streptococcus Pneumoniae Experiment:
What were the experiment groups?
1) Pathogenic CTRL = Treated cells with LIVING S-Strain
2) NON-Pathogenic CTRL = Treated with LIVING R-Strain
3) Non-Pathogenic “Ctrl.” = Treated with HEAT KILLED S-STRAIN (Dead S-Strain)
4) Test (Combo) Grp. = Treated with MIX:
–> LIVING R-STRAIN
–> HEAT-KILLED S-STRAIN
Streptococcus Pneumoniae Experiment:
What was the result of the mice treated with LIVING S-STRAIN?
Mice DIED –> Pathogenic strain killed the mice
Streptococcus Pneumoniae Experiment:
What was the result of the mice treated with LIVING R-STRAIN?
Mice LIVE –> Non-Pathogenic strain did NOT kill mice
Streptococcus Pneumoniae Experiment:
What was the result of the mice treated with HEAT-KILLED S-STRAIN?
Mice LIVE –> Pathogenic strain was DEAD and so it had no pathogenic effect on the mice (didn’t kill the mice)
Streptococcus Pneumoniae Experiment:
What was the result of the mice treated with the COMBO?
–> MIX = Living R-Strain, Heat-Killed S-Strain
Mice DIE
–> Why? Took blood sample to look at the bacteria
Streptococcus Pneumoniae Experiment:
Combo treated mice blood sample results
LIVING S-STRAIN FOUND
(Strange because the mice had been treated with only DEAD S-strain and living R-strain)
Streptococcus Pneumoniae Experiment:
OVERALL RESULT
The R-Strain cells acquired the pathogenicity trait, turning them into S-Strain cells
–> Some chemical component of the pathogenic cells caused this heritable change
Streptococcus Pneumoniae Experiment:
Conclusion and Discovery
Conclusion: R-Strain bacteria had been “transformed” into S-Strain by some heritable factor
Discovery:
Transformation Principle
Streptococcus Pneumoniae Experiment:
What question remained at the end of the experiment?
–> What was the heritable factor that served as the transforming component?
Transformation (Definition as we know it today)
A change in genotype and phenotype due to the assimilation of external DNA by a cell
1944: Avery, MacLeod, McCarty
Oswald Avery, Colin MacLeod, Maclyn McCarty
–> Identified the “transforming substance” from Griffith’s experiment
–> First evidence of DNA as heritable factor (though most were skeptical of this study and did not accept the results as being accurate = DNA debate continued on until 1952)