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DNA
People have always wondered …
How do traits get passed from one
generation to the next?
When did we discover that DNA
existed?
1869- Johann Freidrich Meischer
Found in the nucleus
To simple for heredity
No one was really sure what DNA did
Early 1900’s
Other scientists discover the nucleotide
components
Sugar, nucleotide, nitrogenous bases
Fredrick Griffith - 1928
Transformation experiment
What was he trying to do?
What did he discover?
How was his discovery important?
Oswald Avery- 1944
Avery, MacLeod, McCarty
What were they trying to do?
What did they discover?
How was his discovery important and how
did people react?
Alfred Hershey and Martha Chase-1944
What were they trying to do?
What was their experiment like?
What did they discover?
How was this discovery important?
Now we know lots more
The nature of science
Science builds on itself
Leads to more questions
Technology allows for more discovery
Once people knew DNA was the genetic
material it became a race to discover its
structure
Linus Pauling
Maurice Wilkins and Rosalind Franklin
James Watson and Francis Crick
Why has the discovery of DNA
structure been so important?
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)
He added the
rough strain
And the mouse
lived!
He added the
smooth strain
And the mouse
died
He added the
smooth strain
that had been
killed by heat
And the
mouse lived!
He added smooth
strain killed by heat
with rough strain
And the mouse died.
What happened?
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
Griffith thought that the killed virulent bacteria had
passed on a molecule of inheritance to the nonvirulent bacteria.
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!
Transformation
An organism is transformed by gaining genetic
material from another organism
How was the rough bacteria transformed?
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)
People did not believe his results
The Hershey-Chase Experiment
What is a bacteriophage?
A virus that infects bacteria.
How do viruses work?
The bacteriophage attaches to the surface of
the 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.
What did Hershey and Chase know
about viruses?
1.
2.
3.
Viruses are made of DNA and protein
Bacteriophages infect bacteria and the
cell dies when the new viruses leave
The virus injects its genetic material into
the host cell in order to take it over
4. The purpose of their experiment
Bacteriophages can be used to
determine what the heritable information
of the cell is because they are made of
proteins and DNA
5. The experimental procedure
Radioactive sulfur was
added to the proteins.
Radioactive phosphorous
was added to the DNA
Their final results
6.Conclusion: DNA is the heritable material,
not protein
People finally acknowledge that DNA is 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 was provided without her
knowledge to other researchers named Watson
and Crick. These scientists were also working on
discovering the structure of DNA.
These scientists used her data and that of other
scientists to build their ultimately correct and
detailed description of DNA's structure in 1953.
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.