Embed Size (px)
Citation preview
9/2/2015
1
DNAThe Discovery of Genetic Material
BIOLOGY 12
2 SEPTEMBER 2015
Targets:
� To review the process that lead to discovering that DNA
was responsible for the genetic information in the cell
The Genetic Mystery:� It had become evident that information could be
passed from one generation to the next
� The search began to try and find out what part of
the cell was responsible and how this was
accomplished
The HammerlingExperiment
� The big question: Where is genetic information stored?
� Danish biologist Joachim Hammerling(1930) cut cells into pieces and observed the pieces to see which were able to express hereditary information
� He used a form of green algae (Acetabularia) and amputated the caps to which he found they simply regrew.
� When the foot of the plant was cut, no new foot grew.
� He concluded that the hereditary information residedinthe foot of Acetabularia
The HammerlingExperiment
� He then took two species of
Acetabularia:
� A. mediterranea and A. Crenulata
� He then cut the caps off each
sample and switched a part of the
stalk from one species to the other
� If his theory was correct, regardless
of what species the stalk was from,
the top would be determined by the species of the foot
What did he
find?
Immediately
after the transplant the
cap that regrew
resembled the cap of
the transplanted
stalk
All
Generations after this saw
the cap that matched the
base’s species.
9/2/2015
2
The HammerlingExperiment
� Conclusion
� It was shown through the experiments with Acetabularia that the hereditary information was
stored in the nucleus of the cells in the foot
� This lead biologists to realize that the nucleus is where they needed to focus
The Griffith Experiment
� It became clear that there was something in the
nucleus of the cell that was able to pass information
from one cell to another
� Frederick Griffith (1928) made observations on mice
when infecting them with different strains of the steptococcus pneumoniae bacteria
The Griffith Experiment
(1) He infects
healthy mice
with a pathogenic
(disease
causing) strain of
S.pneumoniae
- The mice die
(2) He infects mice
with a
nonpathogenic strain of the
bacteria (a
mutant form)- The mice Live
The Griffith Experiment
(3) Killing the virus
first to see if
the virus structure itself
was toxic
- The Mice Live
(4) Finally he mixes
dead
pathogenic virus with live
nonpathogenic
virus.- Expected
Mice to live
-Mice Died
The Griffith Experiment
� Conclusion:
� Somehow the information that made the virus deadly was passed from the dead pathogenic virus
to the living nonpathogenic virus
� Genetic information can be passed from dead cells to living ones, transforming them (A process known
as transformation)
The Avery Experiment
� The agent responsible for the transformation of the bacteria in the Griffith experiment was unknown until 1944
� Oswald Avery conducted an experiment:
� A mixture of viruses similar to what was used in the Griffith Experiment was used (Live nonpathogenic and dead Pathogenic S. Streptococcus)
� Almost all (99.98%) of the protein was removed from the virus and the mice were infected again
� The mice died. The lower protein had little effect
� The only material left that could be responsible was DNA (Deoxyribonucleic Acid)
9/2/2015
3
The Hershey-Chase Experiment
� The Avery Experiment’s conclusion that DNA was
responsible for information transmission was not
accepted at first
� Additional Evidence would be provided in 1952
� It was widely believed that protein, not DNA, was
responsible for passing information from one generation to the next.
� Alfred Hershey and Martha Chase conducted an experiment that would settle the argument
The Hershey-Chase Experiment
This side
will track the
Sulfur in the
Protein Coat
This side will
track the phosphorus
in the DNA
The Hershey-Chase Experiment
� Conclusion:
� The Phosphorus (32P) was found in the bacteria, not the sulfur (35S)
� This settles the debate: DNA is definitely what
passes information between generations
https://www.youtube.com/watch?v=BXhFDh15hhg
