Transcript DNA: the Genetic Material Chapter 9.1

DNA: the Genetic Material
Chapter 9.1
Identifying the Genetic Material
Chapter 9 Objectives
Describe the three components of a nucleotide.
 Develop a model of the structure of a DNA
molecule.
 Evaluate the contributions of Chargaff, Franklin,
and Wilkins in helping Watson and Crick
determine the double-helical structure of DNA.
 Relate the role of the base-pairing rules to the
structure of DNA.

What is Genetics?
Genetics – Study of heredity.
 Heredity – Transmission of genetic traits
from parent to offspring.
 Trait – Attribute in an organism controlled
by genes.
 I could do this all day people … mwuah ha
ha ha ha!
 Gene – A section of a chromosome that
codes for a protein or RNA molecule.
 Chromosome – Structure made of DNA &
Protein on which genes are located.

AAAHH HHAAAA!
Chromosome – Structure made of DNA &
Protein on which genes are located.
 I know that for the most part you’re thinking
to yourself, “Uuuhhhh, so what!”
 Well almost 100 years ago this was a
ginormous issue.
 Biologists knew that chromosomes carried
genetic material, but there was something
that they didn’t know which became a rush
against time & other Biologist to discover.

I know you’re all at the edge of your seat!
The question was, what part of a
chromosome actually contained the genetic
material? Is it the DNA or is it protein?
 Biologists couldn’t exactly do genetic testing
until they knew which one of these was
actually carrying the genes.
 For years Biologists worked on finding the
answer. A couple good experiments and a
couple of accidents later an answer was
found.

Frederick Griffith’s Experiments
Griffith was a bacteriologist who was looking for
a vaccine against a bacterium called
Streptococcus pneumoniae.
 S. pneumoniae is the bacteria that causes
pneumonia.
 Vaccine – A substance prepared from killed or
weakened pathogens & introduced to a body to
produce immunity.

Griffith’s 1928 Experiments

Griffith worked with two types of S.
pneumoniae.
A. A strand of S. pneumoniae that was
encased in a capsule made of polysaccharides.
The capsule protects the bacterium from the
body’s immune system. This helps the
bacterium to become a virulent or able to
cause the disease. These bacteria look
smooth so they were called the “S” type.
Griffith’s 1928 Experiments cont.
B. The other type of S. pneumoniae
that Griffith used did not have a capsule,
therefore it did not cause the disease.
These bacterium appeared rough so they
were given the name “R” type.

Griffith did 4 experiments which
accidentally strengthened the argument
that DNA was the genetic carrier.
Four Blind Mice
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1. Griffith first injected mice with the S bacteria.
The S type bacteria was a virulent so it caused
the disease and killed the mouse.

2. Griffith then injected the R bacteria that did
not kill the mouse. The lack of the capsule
caused the mouse’s immune to kill the bacteria.
Four Blind Mice Cont.
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3. Griffith then heated up the S bacteria so that
the DNA would die but the capsule would
remain. Why do you think he did this?

The “heat-killed” bacteria did not kill the mouse.
What does that mean about the capsule?
Four Blind Mice Cont.
4. Just as a gag Griffith took the “heat-killed” S
bacteria and the normal R type mixed them
together and injected that into the mouse.
 What happened?
 The mouse died! What? Why?
 When Griffith looked at the blood of the dead
mouse he noticed that the living R type had
acquired capsules. The harmless R type had
transformed into harmful S types.

Griffith’s Discovery
Griffith discovered what is now called
transformation.
 Transformation - is a change in shape or
identity when bacteria take in foreign
genetic material.
 This “gag” was the first real step in
proving that DNA was the real genetic
carrier.
 Next we find out how it happened!
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Griffith’s Discovery of Transformation
Want more Proof?
In 1944 a Biologist named Oswald Avery
performed an experiment very similar to
that of Griffith.
 Avery used DNA and protein destroying
enzymes in his stock of S. pneumoniae.
 If DNA were the instructions for making
the capsule what would happen to the
bacteria if a DNA destroying enzyme was
introduced?
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Hershey and Chase
In 1952 Alfred Hershey and
Martha Chase performed
an experiment that put a
final nail in protein’s coffin.
 H & C used a virus called a
T2 Bacteriophage (T2
phage) which injects it’s
hereditary material into a
bacteria.
 A T2 phage is made of a
protein sheath that
protects its DNA…that’s it.
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Fun with Radioactive Material
Step 1: They first grew T2 with E. coli in
a medium that contained radioactive
Sulfur (35S). Sulfur attaches to proteins.
They also prepared T2 with E. coli in a
medium that contained radioactive
Phosphorus (32P). Phosphorus attaches to
DNA.
 Step 2: The 35S and 32P were allowed to
infect the E. coli.
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Fun with Radioactive Material Cont.
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Step 3: The 35S and 32P with the E. coli were
thrown into a centrifuge to strip them apart from
each other. The lighter material at the top of
the test tube was the virus, the heavier material
was the bacteria.
H & C Finally!
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H & C noticed that the radioactive DNA
material was with the bacterial material
and the radioactive protein material was
only found with the viral material in the
top part of the test tube.
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What does that mean?