DNA as Genetic Material

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DNA as Genetic Material

• established by several critical experiments
• Fred Griffith (1928)
• Oswald T. Avery, C. M. MacLeod, and M. J. McCarty
  (1944)
• Alfred D. Hershey and Martha Chase (1952)

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Discovery of a transforming principle

• Frederick Griffith, 1928
• - Pneumococcus (Streptococcus pneumoniae)
  infects mice.
• - Mice develop pneumonia and die.
• Two types of bacteria

- S bacteria smooth coat- pneumonia
- R bacteria rough no coat - no pneumonia
Coat type is associated with virulence.
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1944 - Avery, MacLeod McCarty
Purified DNA as transforming factor

• Work not well-received
• Protein more complex better able to store
  information

Oswald Avery
Colin MacLeod
Maclyn McCarty
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1952 - Hershey Chase
Viral DNA (not protein) programs cells
Bacteriophages
Martha Chase Alfred Hershey
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Radioactive protein (35S)
Centrifuge and measure radioactivity in pellet
and supernatant
T2 Phage
Bacterium
Radioactivity in supernatant, but not pellet
Radioactive phage infects bacterial cells
Blender separates protein coats from bacterial
surface
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Radioactive DNA (32P)
Radioactivity in pellet, but not supernatant
Therefore, it is the viral DNA, and not protein,
that specifies all the information needed to
synthesize new virus.
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DNA

• Discovery of the DNA double helix
• A. 1950s
• B. Rosalind Franklin - X-ray photo of DNA.
• C. Watson and Crick - described the DNA
  molecule from Franklins X-ray.

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The Structure of DNA

• Four features summarize the molecular
  architecture of DNA
• The DNA molecule is a double-stranded helix.
• The diameter of the DNA molecule is uniform.
• The twist in DNA is right-handed.
• The two strands run in different directions (they
  are antiparallel).

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The Structure of DNA

• The sugarphosphate backbones of each strand coil
  around the outside of the helix.
• The nitrogenous bases point toward the center of
  the helix.
• Hydrogen bonds between complementary bases hold
  the two strands together.
• A always pairs with T (two hydrogen bonds).
• G always pairs with C (three hydrogen bonds).

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Deoxyribonucleic Acid (DNA)

• Made up of nucleotides (DNA molecule) in a DNA
  double helix.
• Nucleotide
• 1. Phosphate group
• 2. 5-carbon sugar
• 3. Nitrogenous base
• 2 nm wide

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DNA Nucleotide

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DNA Double Helix

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Nitrogenous Bases

• PURINES
• 1. Adenine (A)
• 2. Guanine (G)
• PYRIMIDINES
• 3. Thymine (T)
• 4. Cytosine (C)

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BASE-PAIRINGS

• Base of
• Purines Pyrimidines Pairs H-Bonds
• Adenine (A) Thymine (T) A T 2
• Guanine (G) Cytosine (C) C G 3

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Chargaffs Rule

• Adenine must pair with Thymine
• Guanine must pair with Cytosine
• Their amounts in a given DNA molecule will be
  about the same.

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Question

• If there is 30 Adenine, how much Cytosine is
  present?

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• There would be 20 Cytosine.
• Adenine (30) Thymine (30)
• Guanine (20) Cytosine (20)
• (50) (50)

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Synthesis Phase (S phase)

• S phase in interphase of the cell cycle.
• Nucleus of eukaryotes

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Figure 11.7 Base Pairing in DNA Is Complementary
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• The genetic material performs four important
  functions
• It stores all of an organisms genetic
  information.
• It must be precisely replicated in the cell
  division cycle.
• It is expressed as the phenotype.
• It is susceptible to mutation.

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Watson Crick predicted that each DNA strand
could serve as a template for the replication of
a new strand
Q What is the mode of replication?
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Three possible modes of replication
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The Meselson-Stahl Experiment

• The Meselson-Stahl experiment
  demonstrated that replication is
  semiconservative
• This experiment took advantage of
  the fact that nucleotide bases contain
  nitrogen
• Thus DNA contains nitrogen

• The most common form of Nitrogen is N14 with 7
  protons and 7 neutrons
• N15 is called heavy nitrogen as it has 8
  neutrons thus increasing its mass by 1 atomic
  mass unit

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The Meselson-Stahl Experiment
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The conservative and dispersive models make
predictions that do not come true thus, by
deduction, the semi-conservative model must be
true.
Prediction after 2 or more replications
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