DNA

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DNA

• As the Genetic Material

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Topics

• Evidence of DNA as genetic material
• The Watson-Crick Model
• DNA Replication

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DNA as genetic material

• Chemical Characterization
• Bacterial Transformation
• Evidence That Genes are DNA

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Deoxyribonucleic Acid

• Phosphate
• Deoxyribose Sugar
• Purine or Pyrimidine Base

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3
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H-bonds
Deoxyribose
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Sugar-P Backbone
A T G C
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Bacterial Transformation Implicates DNA as
Genetic Material
Griffith, 1928
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Hershey-Chase
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Hershey-Chase Experiment
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Erwin Chargaff
Note 11 ratio
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The Watson-Crick Model

• History
• Complementary Base Pairing
• Structure and Function

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Read extensively the works of Griffith, Avery,
Hershey-Chase, and Chargaff
Obtained detailed photograph of X-ray
Diffraction pattern of DNA Helix
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Complementary Base-Pairing
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Most are B-form
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It has not escaped our notice that the specific
pairing we have postulated immediately
suggests a possible copying mechanism for the
genetic material.
Watson Crick, 1953 Nature
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DNA Replication

• Proof of Semi-conservative Replication
• Molecular mechanism of Replication

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Semi-Conservative Replication
Parent Strand
Parent Strand
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Meselson Stahls Experimental Method

• Isolated DNA from cells grown in different
  nitrogen-isotope cultures
• Subjected cultures to density gradient
  centrifugation (gt50,000 rpm)
• DNA containing 15N will be lower band
• DNA containing 14N will be higher band

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Meselson Stahls Experimental Method

• Cells from 15N growth medium transferred to 14N
  medium
• Collected after 1 round of replication (20 min.)
• Collected again after 2 rounds of replication
• Centrifuge and see where the bands are

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Mechanics of DNA Replication

• Initiation
• Elongation

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Mechanics of DNA Replication

• Molecules involved
• Nucleotides
• Initiator Protein
• DNA Helicase
• DNA Polymerase III
• RNA Primer
• DNA Ligase

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Step 1 - Initiation

• DNA unwinds at the origin of replication
• Two complementary strands separate
• How?
• Initiator protein binds to the region
• Attracts DNA Helicase
• DNA Helicase catalyzes the unwinding

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DNA Helicase
Template
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DNA Polymerase III

• Copies only unwound DNA
• Adds only DNA to the end of an existing chain
• Functions in only one direction
• 5 to 3

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RNA Primer Provides END to which DNA
Polymerase can add nucleotides
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Step 2 Elongation

• DNA Polymerase can now go to work
• Again in one direction only
• Binds and links together DNA subunits
  (nucleotides)
• Subunits linked by phosphodiester bonds

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A nucleotide triphosphate
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Molecular Model of Crossing Over?
Not Responsible for