Chapter 12 DNA

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Chapter 12DNA RNA
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12-1 DNA

• How do genes work?
• What are they made of?
• How do they determine characteristics of
  organisms?
• In the middle of the 1900s, questions like these
  were on the minds of biologists everywhere.

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Griffith and Transformation

• 1928 British scientist Frederick Griffith was
  trying to figure out how bacteria make people
  sick.
• Griffith wanted to learn how certain types of
  bacteria produce a serious lung infection known
  as pneumonia.

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Frederick Griffith
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What did he do?

• isolated two slightly different strains of
  pneumonia bacteria from mice
• Both strains grew very well, but only one of the
  strains caused pneumonia.
• disease-causing strain of bacteria smooth
  colonies
• harmless strain colonies with rough edges

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Griffith's Experiments

• injected mice with the S bacteria ? pneumonia and
  died.
• mice injected with R bacteria ? totally healthy.
• Griffith wondered if the disease-causing bacteria
  might produce a poison.

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• Heat killed S bacteria into mice ? Healthy mice!
• The mice survived, suggesting that the cause of
  pneumonia was not a chemical poison released by
  the disease-causing bacteria.
• Heat killed S bacteria R bacteria in mice ?
  DEAD mice!
• He found their lungs filled with the deadly S
  bacteria.
• Some factor from the dead bacteria had
  transformed the harmless bacteria into
  disease-causing ones.

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Transformation 

• process in which one strain of bacteria is
  changed by a gene or genes from another strain of
  bacteria
• Griffith hypothesized since the ability to cause
  disease was inherited by the transformed
  bacteria's offspring, the transforming factor
  might be a gene (genetic material)

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• Although Griffith made a great discovery, he did
  not follow through and figure out what actually
  transformed the bacteria. 
• Avery came along and did that.

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Avery and DNA

• 1944, a group of scientists led by Canadian
  biologist Oswald Avery at the Rockefeller
  Institute in New York decided to repeat
  Griffith's work.
• to determine which molecule was the genetic
  material responsible for transformation.

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• Oswald Avery

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Averys experiment

• Made an extract from the heat killed bacteria
• Used enzymes that destroy the lipids,
  carbohydrates, proteins, and RNA.
• Transformation still occurred.
• Used DNA destroying enzyme.
• Transformation did NOT occur.
• DNA must be the genetic material.
• Avery and other scientists discovered that the
  nucleic acid DNA stores and transmits the genetic
  information from one generation of an organism to
  the next.

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• Scientists are skeptical.
• Scientists dont always believe things with proof
  from only one experiment
• So other scientists set out to prove what the
  genetic/hereditary information of an organism is.

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Hershey Chase

• Alfred Hershey and Martha Chase
• 1952, American scientists
• Studied viruses (non living particles)

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Martha Chase and Alfred Hershey
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The Hershey-Chase Experiment

• Used bacteriophages (virus that infects bacteria)
• composed of a DNA or RNA core and a protein coat
• When a bacteriophage attacks a bacterium, it
  injects its genetic information into the
  bacterium.
• Those genes take over the cell, producing many
  new viruses.

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Bacteriophage
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Nucleic Acid or protein??

• They wanted to find out which part of the phage
  (DNA or protein), produced new phages.
• Grew bacteriophages with radioactive markers
• phosphorus-32 (32P) only in DNA
• sulfur-35 (35S) only in Protein

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• mixed the marked viruses with bacteria
• waited a few minutes for the viruses to inject
  their genetic material.
• separated the viruses from the bacteria and
  tested the bacteria for radioactivity.
• Nearly all the radioactivity in the bacteria was
  from phosphorus (32P), the marker found in DNA.

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• Hershey and Chase concluded that the genetic
  material of the bacteriophage was DNA, not
  protein.

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• The
• Components
• and Structure
• of DNA

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DNA components

• DNA is a long molecule made up of units called
  nucleotides.
• Nucleotides are made up of three basic
  components
• 5-carbon sugar (deoxyribose)
  
• a phosphate group
• a nitrogenous base
• 4 kind of nitrogen bases found in DNA
• adenine (A), guanine (G) double ringed
  (purines)
• cytosine (C), thymine (T) single ringed
  (pyrimidines)

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DNA structure

• backbone of a DNA chain sugar and phosphate
  groups of each nucleotide.
• The nitrogenous bases stick out sideways from the
  chain.
• The nucleotides can be joined together in any
  order
• any sequence of bases is possible.
• With 4 bases millions of different combinations
  are possible

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DNA
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Erwin Chargaff

• American biochemist, 1940s
• discovered that the percentages of guanine G
  and cytosine C bases are almost equal in any
  sample of DNA
• Later found that A T
• Scientists had NO idea what this was

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Chargaff's Rules 
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Franklin Wilkins

• British scientists, 1952
• used a technique called X-ray diffraction to get
  information about the structure of the DNA
  molecule
• X-shaped pattern shows that the strands in DNA
  are twisted around each other like the coils of a
  spring
• a shape known as a helix
• the X suggests that there are two strands in the
  structure
• Other clues suggest that the nitrogenous bases
  are near the center of the molecule

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Franklin

• Wilkins

X-shaped DNA (from X-ray Diffraction
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Watson Crick

• Francis Crick, a British physicist
• James Watson, an American biologist
• trying to understand the structure of DNA
• by building three-dimensional models of the
  molecule
• 1953, they are shown a picture of Franklins
  x-ray and immediately knew the structure
• Watson and Crick's model of DNA was a double
  helix, in which two strands were wound around
  each other.

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Double Helix

• Twisted ladder or spiral staircase
• realized that the double helix accounted for many
  of the features in Franklin's X-ray pattern
• did not explain what forces held the two strands
  together.
• discovered that hydrogen bonds could form between
  certain nitrogenous bases and provide just enough
  force to hold the two strands together

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Base Pairing

• hydrogen bonds can form only between certain base
  pairs
• adenine (A) with thymine (T)
• guanine (G) with cytosine (C)

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