Transcript Discovery of DNA

Discovery of DNA
Fredrick Griffith – 1928
Oswald Avery – 1944
Alfred Hershey & Martha Chase - 1952
Griffith
S – Strain virulent (contains capsule –
causes the disease)
 R – Strain non-virulent (no capsule – does
not cause disease

Griffith – Background Information
Studied Streptococcus pneumoniae
 Attempted to make vaccine for pneumonia
◦ 2 strains
R cells (rough)
S cells (smooth)

Griffith – Experiment / Results
E 1: Inject mice w/ live R cells  mice LIVE
 E 2: Inject mice w/ live S cells  mice DIE
 E 3: Inject mice w/ heat killed S cells  mice
 LIVE
 E 4: Inject mice w/ heat killed S cells and live
R cells  mice DIE

◦ Heredity material from S cells transform R cells
into S cells.
Griffith Conclusion

Transformation – transfer of genetic material
from 1 cell to another or 1 organism to
another.
Avery Background Information

Tested to see if the transforming agent in
Griffith’s experiment was Protein, DNA, or
RNA
Avery’s Experiment
E 1 – used protease to destroy proteins in
heat killed S cells
 E 2 – used RNA ase to destroy RNA in heat
killed S cells
 E 3 – used DNA ase to destroy DNA in
heat killed S cells
Separately mixed 3 batches of heat killed S cells
with live R cells and injected the mice

Avery’s Results
Cells missing proteins and RNA were able
to transform R cells into S cells and kill the
mice
 Cells missing DNA did NOT transform R
cells into S cells and mice lived.

Avery’s Conclusion

DNA is the transforming agent in BACTERIA.
DNA is responsible for transformation in
bacteria
Hershey / Chase Background Info

Tested whether DNA or proteins were
heredity material viruses transfer when
viruses enter a bacterial cell

Bacteriophage (phage) – virus that infects
bacterial cells.
Hershey/Chase Experiment
Used radioactive isotopes to label proteins
and DNA in “phage”.
 Radio Sulfur to label proteins
 Radio Phosphorus to label DNA

◦ Allowed protein labeled and DNA labeled phages
to infect E-coli (bacteria) and tested E-coli for
the radioisotopes.
Hershey/Chase Results

They found ALL viral DNA and little of the
protein had entered E-coli cells
Hershey/Chase Conclusion

That DNA is the heredity material in
viruses.