Transcript The Avery and Hershey-Chase Experiments

The Avery and HersheyChase Experiments:
The Active Principle is DNA!
LeAnne, Carly, and Drabe.
The Avery Experiments
• Avery provided conclusive evidence that DNA is
the heredity material for the bacterial specimens
under investigation.
– Oswald Avery (with his co-workers MacLeod and
McCarty) characterized what they called the
“transforming principle” from Griffith’s experiment.
– They prepared a mixture of dead S Streptococcus
and live R Streptococcus. (That Griffith had used).
– Avery and his colleagues achieved 99.98% purity by
removing as they could form their mixtures.
– The transforming activity was NOT reduced.
Avery Experiments (cont.)
– The properties of the transforming principle
resembled those of DNA in several ways:
• When the purified principle was analyzed chemically, the
array of elements agreed closely with DNA
• When spun at high speeds, the transforming principle
migrated at the same level (density) as DNA
• Extracting the lipid and protein from the purified transforming
principle did not reduce its activity
• Protein-digesting enzymes did not affect the principle’s
activity, nor did RNA-digesting enzymes
• The DNA-digesting enzyme DNase destroyed all
transforming activity
Avery Experiment: Conclusion
• The researchers concluded that “a nucleic
acid of the deoxyribose type is the
fundamental unit of the transforming
principle of Pneumococcus Type III” –
basically:
– DNA is the hereditary material for this
bacterial species.
The Hershey-Chase Experiment
• Hershey and Chase provided further
evidence that heredity material in
bacteriophages was found in DNA, not in
proteins.
– Many did not accept Avery’s conclusions until
1952 when Alfred Hershey and Martha Chase
conducted this experiment with
bacteriophages (viruses that attack bacteria)
The Hershey Chase Experiment
• In the Hershey-Chase experiment, bacterial
viruses called phage, were used to
demonstrate that DNA is the genetic material.
The phage used in this experiment consisted
of a DNA molecule surrounded by a protein
coat.
• When phage infect bacteria, they attach to
the surface of the bacterium and inject the
DNA into the cell. The protein coat remains
on the outside of the cell.
Hershey-Chase (cont.)
• In the first part of the experiment, phage were
produced in a medium containing S-35
radioactively labeled amino acids. This resulted
in a phage population with S-35 labeled proteins
but no radioactive label in the DNA
• The phage were then allowed to infect the
bacteria.
• They attached to the bacterial cell and injected
their DNA, but the radioactively-labeled protein
coat remained on the outside of the cell
Hershey-Chase (cont.)
• The phage produced in these cells
contained no radioactivity.
• Vigorous shaking caused the empty
protein coats to be removed, but did not
interfere with production of new phage.
• In the second part, phage were produced
in a medium containing P-32 labeled
deoxyribonucleotides. This resulted in
phage population with P-32 labeled DNA,
but no radioactive label in the protein.
Hershey-Chase (cont.)
• When the phage infected the bacteria, the
P-32 labeled DNA entered the cell and
could not be found in phage subsequently
produced in the infected bacteria.
• This demonstrated that the DNA, but not
the protein, carries the genetic information
for a new generation of phage! SWEET!