The Genetic Code

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The Genetic Code

• The word genetic refers to heredity.
• A code is a set of symbols or signals used to
  pass on information.
• The genetic code is the way in which cells
  store information that is passed from one
  generation to the next generation.

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The Search for the Genetic Code

• Frederick Griffith He studied bacteria that
  caused pneumonia in mice.
• In 1928 he did the following experiment.

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Transformation Of BacteriaTwo Strains Of
Streptococcus
Rough Strain (Harmless)
Smooth Strain (Virulent)
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Transformation Of BacteriaThe Griffith Experiment
OUCH!
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The Search for the Genetic Code
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The Search for the Genetic Code

• Results from 4 show that the heat-killed strain
  had passed to the live harmless strain its
  disease-causing ability.
• That is, one form of bacteria was transformed
  into another form.
• Griffith called this process transformation.
• He hypothesized that a factor was transferred
  from the heat killed cells into the live cells

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The Search for the Genetic Code

• Oswald Avery and others In 1944 they repeated
  Griffiths experiment to see which molecules
  were responsible for the transformation.
• From the heat-killed bacteria they made an
  extract or juice.
• Individually they treated the extract with
  enzymes that destroy lipids, proteins,
  carbohydrates, and RNA.

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The Search for the Genetic Code

• The results were that transformation still
  occurred and so none of these molecules were
  the transferring agent.
• They repeated the experiment with enzymes that
  break down DNA (deoxyribonucleic acid).
• The results were that transformation did not
  take place.

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Yes
Lippase
Yes
Protease
Yes
Saccharase
No
Nuclease
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The Search for the Genetic Code

• Conclusion DNA is the transforming material and
  is the molecule that stores and transmits the
  genetic code from one generation of an organism
  to the next.

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SEARCH FOR THE GENETIC CODE CONTINUES

• After the Oswald Avery experiment some scientists
  still remained skeptical that DNA was the
  genetic material.
• In 1952 the following experiment was done by
  Alfred Hershey and Martha Chase.

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The Hershey-Chase Experiment

• They used bacteriophages which are viruses that
  infect bacteria (e.g., T2 phage).
• They are composed of a DNA core and a protein
  coat.
• They infect bacteria by attaching to the
  surface and injecting material.

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The Hershey-Chase Experiment

• The injected material is used to make many
  copies of the virus.
• Viral copies are produced until they cause the
  bacteria to burst, releasing the virus.
• Because the injected material is used to make
  copies, it must be the genetic code.
• Important question What is being injected from
  the virus, the protein coat or DNA core?

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The Hershey-Chase Experiment

• They labeled the protein coat of the virus with
  radioactive sulfur-35.
• They labeled the DNA core of the virus with
  radioactive phosphorus-32.
• They mixed the labeled viruses with the
  bacteria, waited for injection, then
  separated the parts using a blender and
  centrifuge.

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The Hershey-Chase Experiment

• Hershey and Chase took advantage of the fact that
  T2 phage is made of only two things Protein and
  DNA

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Using S35
When centrifuged, phage protein coats remain in
the supernatant while bacteria form a pellet
The supernatant is radioactive, but the pellet is
not.
Did protein enter the bacteria?
Is protein the genetic material?
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Using P32
When centrifuged, phage protein coats remain in
the supernatant while bacteria form a pellet
The pellet is radioactive, but the supernatant is
not.
Did DNA enter the bacteria?
Is DNA the genetic material?
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The Hershey-Chase Experiment
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The Hershey-Chase Experiment

• Conclusion
• The Hershey-Chase experiment showed definitively
  that DNA is the molecule that carries the genetic
  code.

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AN IMPORTANT QUESTION REMAINED WHAT IS THE
STRUCTURE OF DNA?

• In the early 1950s Rosalind Franklin Maurice
  Wilkins took x-ray pictures of the DNA molecule.
• These x-ray pictures provided important clues
  about the structure of DNA.
• For example, the fibers that make up DNA are
  twisted and the molecules in the fibers are
  spaced out in regular intervals.

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AN IMPORTANT QUESTION REMAINED WHAT IS THE
STRUCTURE OF DNA?

• Another important piece of information on the
  structure of DNA was provided by Erwin
  Chargaff.
• He observed that in any sample of DNA, the
  number of
• Adenine molecules (A) (T) Thymine molecules
• Cytosine molecules (C) (G) Guanine molecules