Transcript Discovering the material for heredity: DNA

Discovering the
material for heredity:
DNA
Ch. 13 Biology In Focus
AP Biology 2014
Time Line
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Frederick Griffith (1928) Griffith discovers that bacteria can change from
one form to another.
Oswald Avery & colleagues (1944) follow up Griffith’s earlier discovery
and conclude that the transforming factor is DNA.
Rosalind Franklin & Maurice Wilkins (1950) provide evidence that DNA
is in the form of a double helix.
Edwin Chargaff (1951) publishes that the nitrogenous bases of DNA
occur in a ratio, with equal amounts of adenine and thymine, and
cytosine and guanine.
Alfred Hershey & Martha Chase (1952) Hershey and Chase conduct
experiments proving that DNA was the hereditary material.
James Watson & Francis Crick (1953) publish the three dimensional
structure and composition of DNA.
“Ancient” History
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By the 1920's chromosomes were suspected as
carriers of genetic information based on
observations of mitosis.
Biochemical studies of chromosome
composition demonstrated that they were
composed of 30 - 50% nucleic acid and 50 70% protein.
It was generally believed that PROTEINS
would prove to be the carriers of genetic
information. WHY?
Genetic Transformation Discovered
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Fred Griffith unwittingly discovered
transformation. He showed that some
“active genetic substance” could be
transferred from a dead bacteria capable of
causing disease to a live harmless bacteria
– making this bacteria dangerous.
How did he accomplish this?
Griffith’s Experiment
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Griffith was attempting to develop a
vaccine for Streptococcus pneumoniae.
There were two strains of Streptococcus,
one of which was harmless to people. The
other strain caused pneumonia. The term
for the ability of an organism to cause
disease is virulence.
More on Streptococcus…
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Strain #1: S strain
– Was called S strain
because it formed
smooth colonies on a
petri dish culture.
– Had a polysaccharide
coat that protected it
from attack by the
immune system.
– Was virulent.
Still More on Streptococcus…
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Strain #2: R strain
– Was called R strain because it formed rough
colonies on a petri dish culture.
– Did not have the polysaccharide coat that
protected it from attack by the immune
system.
– Was avirulent (harmless).
Griffith and later Oswald Avery & his
colleagues perform the following
experiment…
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Live S strain injected into a mouse yields a
dead mouse.
Live R strain injected into a mouse yields
a live mouse.
BUT… what if you heat kill the S strain
and add it to the live R strain?
“Something” (we now know this to be DNA)
from the virulent S strain of bacteria had been
able to transform the harmless R strain into a
mouse killer!
Avery & colleagues…
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Avery and his colleagues repeated Griffith’s
experiment but added an additional step.
First they added a protein-destroying enzyme to
the heat-killed S strain. Mice still died.
They repeated the experiment but the second
time added a DNA-destroying enzyme to the
heat-killed S strain. The mice didn’t die!
The “transforming factor” had to be
DNA!!
Alfred Hershey & Martha Chase
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Even after Avery’s
experiments, scientists were
still skeptical about the
possibility that DNA was the
“stuff of heredity.”
In 1952, Alfred Hershey &
Martha Chase performed an
elegant series of experiments
which proved that DNA was
the genetic material – using a
household blender!
Hershey and Chase used?
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Hershey and Chase used the bacteriophage, a virus which
infects and kills bacterial cells.
Viruses infect living cells and then multiply inside these
cells, producing millions of copies of the virus which
then explode the cell, releasing these copies to go out and
infect more cells.
Experimental Predictions
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If the virus carried the instructions for making
copies of itself (its genetic material) in the form
of protein, then the virus would have to inject its
protein into the bacteria.
If the virus carried the instructions for making
copies of itself (its genetic material) in the form
of DNA, then the virus would have to inject its
DNA into the bacteria.
The Experiment
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Use a batch of virus with radioactive protein
shells (35S).
Use a second batch of virus with radioactive
DNA (32P).
Allow each to infect bacteria (E. coli), then
remove the virus on the outside using a blender.
Collect the E. coli bacteria and see whether they
contain 35S (radioactive protein) or 32P
(radioactive DNA) on the inside.
Results
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E. coli bacteria in the pellet contained
virtually no 35S.
The offspring of the virus (its progeny)
contained lots of 32P-labelled DNA.
Conclusion: DNA carried the genetic
information!!
What did we know
about DNA?
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We knew that it was composed of chains
of four nucleotides – containing four
different nitrogenous bases.
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Adenine
Thymine
Cytosine
Guanine
What is a nucleotide?
Nitrogenous Bases
One strand = nucleotides covalently
linked, forming a sugar-phosphate
backbone
Purines & Pyrimidines
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The bases with one ring
are pyrimidines.
– Cytosine
– Thymine
The bases with two rings
are purines.
– Adenine
– Guanine
Chargaff’s Rule
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1949 – Edwin Chargaff noticed that in every
analysis of DNA that he performed, the
amount of adenine present always equaled
the amount of thymine and that the amount
of cytosine always equaled the amount of
guanine.
There could be different overall amount of C
& G and A & T – but these pairs of bases were
always present in equal ratios!
This is called Chargaff’s Rule.
Rosalind Franklin &
Maurice Wilkins
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Rosalind Franklin, a
talented X-ray
crystallographer
working with Maurice
Wilkins, developed Xray diffraction images
of DNA.
Her data indicated that
DNA existed in the
form of a double helix.
James Watson
& Francis Crick
What they knew…
Chargaff’s Rule: Adenine &
thymine occur in equal amounts, as
does cytosine and guanine.
 Rosalind Franklin’s data indicated
that the molecule was a double helix.
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What they finally figured out…
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DNA is a two-stranded double helix - like
a twisted ladder!
The legs of the ladder are composed of the
phosphates and deoxyribose sugars of the
nucleotides (phosphate-sugar backbone).
The rungs are composed of the
nitrogenous bases, which stick together by
hydrogen bonding.
A hydrogen bonds with T and G hydrogen
bonds with C
Base-pairing between the
two strands of DNA
Yeah DNA!!
DNA structure cliff notes
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Composed of two chains, or strands,
of nucleotides (A, T, C and G)
The nucleotides in one strand are
connected by covalent bonds
The two strands are held together by
weak hydrogen bonds – A bonds with
T and C bonds with G
In cells, the DNA curls around forming
a double-helix – but we often visualize
DNA as a flat ladder because it is a
little easier to see the nucleotides…
Strands run in opposite
directions = antiparallel