DNA
1. Griffith Experiment


The genetic material in cells, was not clear to
scientists for many years.
Remember Chromosomes consist of
proteins.
OR
Scientists were trying to figure which one out
of DNA or protein played the role of the
genetic material. . . ?
This led to several experiments, two of which
are very important.
Griffith Transformation Experiment


He was trying to find a vaccine against
Streptococcus pneumonia (bacteria that
caused pneumonia)
Isolated two different strains of bacteria


Rough surface appearance (R); no
disease/harmless (no coating)
Smooth surface appearance (S); disease
causing/pathogenic (coating)
Set up four experiments




Live R injected=mice lived
Live S injected=mice died
Dead S injected=mice lived
Live R + dead S=mice died
Why? Griffith thought that the killed virulent
bacteria had passed on a characteristic to the
non-virulent one to make it virulent. He thought
that this characteristic was in the inheritance
molecule. This passing on of the inheritance
molecule was what he called transformation.

Griffith found from this experiment that
even though he had killed the S cells, he
hadn’t destroyed their hereditary material,
which was the one part that caused the
disease! When some more experiments
had been done, it had been discovered that
the harmless R cells, had used the
information from the hereditary material of
the dead S cells and became harmful; this
he called, hereditary transformation.
Transformation

Organism transformed by gaining genetic material
from another organism

The dead genes of the smooth strand, transfered
their DNA to the rough strand. The rough strand
thus gained the instructions to produce a smooth
and deadly coat. Since the genes that were
transfered were dead, it is evident that the rough
strand only became deadly because of the DNA it
gained.
Oswald Avery 1944


Continued work of Griffith and found that
extracts of killed bacteria could transform
healthy bacteria cells
Showed the heritable material in extracts
was DNA (stores and transmits genetic
information)
Hershey & Chase Experiment
What they know (write this down)
1.
2.
3.
viruses have DNA inside a protein
coat/shell called a capsid
Viruses which infect bacterium is a
bacteriophage because the host
bacterium cell is killed as the new viruses
leave the bacterial cell.
To do this, the virus must inject its genetic
code into the host cell.
What is a bacteriophage?

A virus that infects bacteria.

With its six legs, the
bacteriophage
attaches to the
surface of the much
larger bacteria
Escherichia coli (E.
coli).


Once attached, the
bacteriophage
injects DNA into the
bacterium. The DNA
instructs the
bacterium to
produce masses of
new viruses.
So many are
produced, that the E.
coli bursts.
The Big Question? (write this down)
Bacteriophages consist of both protein and
DNA.
4. So, did proteins, DNA, or both contain
the viral genetic information?
Procedure
5. They used E. coli as the experimental
bacteria and a virus called T2.
 Next, they injected sulfur into the protein
coat and phosphorus into the DNA core to
make them radioactive allowing scientist to
track down if the radioactive sulfur protein
coating or phosphorus was passed on or
not.
Results
6.The DNA, not the protein, is its genetic
code.
Conclusion
7.People finally acknowledged DNA as the
genetic material!!
Erwin Chargaff 1949


Studied composition of DNA
The amount of adenine always equals the
amount of thymine and the amount of
cytosine always equals the amount of
guanine.
A=T
C=G
Rosalind Franklin


X-ray diffraction images of DNA fibers
Position of atoms can be calculated from
images

Franklin's data and her unpublished conclusions
at a routine seminar, her work was provided without Randall's knowledge - to her competitors
at Cambridge University, Watson and Crick. The
scientists used her data and that of other
scientists to build their ultimately correct and
detailed description of DNA's structure in 1953.
Franklin was not bitter, but pleased, and set out
to publish a corroborating report of the WatsonCrick model. Her career was eventually cut short
by illness. It is a tremendous shame that
Franklin did not receive due credit for her
essential role in this discovery, either during her
lifetime or after her untimely death at age 37 due
to cancer.
James Watson & Francis Crick 1953

Put together model for DNA structure; double
helix (two strands of DNA bonded together wound
in a helix)
Sugar-phosphate backbone (from the
outside of the molecule)
Nitrogenous base form the middle of
the molecule
purines bond with pyrimidines
Won Nobel prize for their discovery!
Answer the Questions
1.
2.
3.
Explain what is meant by the term
transformation. Why are Griffiths
experiments on S. pneumococci
considered to be so important?
What's a bacteriophage How does it
infect bacteria?
Describe the procedure by which Hershey
& Chase demonstrated that DNA is the
genetic material.