Chapter 9 DNA: The Molecule of Heredity

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Chapter 9 DNA The Molecule of Heredity
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Contributors to the field of DNA

• Gregor Mendel Frederick Griffith Oswald Avery
• Hershey and Chase Erwin Chargaff Maurice
  Wilkins and Rosalind Franklin James Watson and
  Francis Crick

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Gregor Mendel

• Hypothesized that in every generation, a plant
  inherits two units of information for a trait,
  one from each parent
• Pea plant experiments
• Without this information about inheritance, the
  idea of heredity would never have developed
• Without the idea of heredity, nobody would know
  about DNA

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Frederick Griffith

• 1928
• Searching for a vaccine against Streptococcus
  pneumoniae
• He did four experiments in which he injected
  strands of bacteria into mice, one strand that
  was harmless (R) and one that was harmful (S).

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Frederick Griffiths Experiments

• Injected the live R bacteria cells into the mice
• Mice lived.

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Frederick Griffiths Experiments

• Injected the live R bacteria cells into the mice
• Mice lived.
• Injected the live S bacteria cells into the mice
• Mice died.

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Frederick Griffiths Experiments

• Injected the live R bacteria cells into the mice
• Mice lived.
• Injected the live S bacteria cells into the mice
• Mice died.
• Killed the harmful S cells with extreme heat, and
  then injected the dead S cells into the mice
• Mice lived.

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Frederick Griffiths Experiments

• Injected the live R bacteria cells into the mice
• Mice lived.
• Injected the live S bacteria cells into the mice
• Mice died.
• Killed the harmful S cells with extreme heat, and
  then injected the dead S cells into the mice
• Mice lived.
• Added live R cell (which are harmless) to the
  already dead heat-killed S cells, and then
  injected it into the mice
• Mice died!

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Frederick Griffiths Experiments

• Even though the S cells were dead the hereditary
  material had not been destroyed
• This was the one part that caused the disease!

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Frederick Griffith More Experiments

• The harmless R cells, had used the information
  from the hereditary material of the dead S cells
  and became harmful
• (Oh and by the way he never did find the
  vaccine.)

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Oswald Avery

• 1940s
• Fascinated by Griffiths discovery
• With colleagues found a way to extract the
  heat-killed disease carrying cells
• Reported that DNA, not proteins was the
  hereditary substance in these extracts

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Oswald Averys Experiments

• Protein-digesting enzymes were added to some of
  the extracts (i.e. S strain bacteria)
• The cells were still transformed
• An enzyme that broke up DNA but not the protein
  was added to some of the extracts
• The hereditary transformation was blocked

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The Long Hill

• Many still believed that proteins were the
  primary molecule of inheritance
• Argument Averys findings were probably only
  true for bacteria

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Reproduction of Viruses

• Viruses consist of a protein coat (capsid)
  surrounding a nucleic acid core
• Bacteriophages are viruses that infect bacteria
• Hershey and Chase
• Radioactively labeled the DNA core and protein
  capsid of a phage
• Results indicated that DNA, not the protein,
  enters the host
• The DNA of the phage contains genetic information
  for producing new phages

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Bacteria and Bacteriophages
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Hershey and Chase

• Used viruses (bacteriophages)
• Labeled bacteria with radioisotopes
• 35Sulfur
• 32Phosphorus

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Erwin Chargaff

• Biochemist
• Figured out the equation for the different
  nitrogen-containing bases
• The amount of Adenine will always equal the
  amount of Thymine
• The amount of Guanine will always equal the
  amount of Cytosine.

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Maurice Wilkins and Rosalind Franklin

• The first to obtain very good x-ray diffraction
  images of the DNA fibers
• Franklin determined
• DNA exists as a long, thin molecule of uniform
  diameter
• The structure is highly repetitive
• DNA is helical

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X-Ray Diffraction of DNA
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James Watson and Francis Crick

• 1951
• Using previous X-ray diffraction photos of DNA
  fibers taken by Maurice Wilkins and Rosalind
  Franklin, they discovered that it showed an X
  shape
• Constructed a model of DNA
• Double-helix model is similar to a twisted ladder
• Sugar-phosphate backbones make up the sides
• Hydrogen-bonded bases make up the rungs
• Received a Nobel Prize in 1962

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DNA

• Deoxyribonucleic acid
• Molecule of inheritance of cells and some viruses

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Components of DNA

• Sugar
• Deoxyribose
• Phosphate
• Nitrogen-containing bases
• Adenine
• Cytosine
• Guanine
• Thymine

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Base Pairings

• A T
• C G

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• Pyrimidine
• Single ring
• structure

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Purine Double ring structure
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The DNA Code

• The genetic code is analogous to languages, where
  small sets of letters combine in various ways to
  make up many different words
• English has 26 letters
• The binary language of computers uses only two
  letters (0 and 1, or on and off)

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The DNA Code

• The sequence of only four nucleotides can produce
  many different combinations
• A 10 nucleotide sequence can code for greater
  than 1 million different combinations

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DNA Replication

• All cells come from pre-existing cells
• Cells reproduce by dividing in half
• Each of two daughter cells gets an exact copy of
  parent cells genetic information
• Duplication of the parent cell DNA is called
  replication

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DNA replication

• DNA replication begins when DNA helicases
  separate the two strands
• Hydrogen bonds between bases are broken
• A second strand of new DNA is synthesized along
  each separated strand by DNA polymerases, which
  position free nucleotides across from
  complementary nucleotides

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DNA replication

• Base pairing is the foundation of DNA replication
• An adenine on one strand pairs with a thymine on
  the other strand a cytosine pairs with guanine
• If one strand reads ATG, the other reads TAC
• The two resulting DNA molecules have one old
  parental strand and one new strand
  (semiconservative replication)

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Replication And Proofreading

• During replication, DNA polymerase mismatches
  nucleotides once every 10,000 base pairs
• DNA repair enzymes proofread each new daughter
  strand, replacing mismatched nucleotides
• However

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Mistakes Happen

• DNA is damaged in a number of ways
• Spontaneous chemical breakdown at body
  temperature
• Certain chemicals (some components of cigarette
  smoke)
• UV light from the sun causes DNA damage
• DNA damage leads to uncontrollable cell division
  and skin cancer

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Types of Mutations

• Point mutation - individual nucleotide in the DNA
  sequence is changed
• Insertion mutation - one or more nucleotide pairs
  are inserted into the DNA double helix
• Deletion mutation - one or more nucleotide pairs
  are removed from the double helix