DNA Structure and Functions

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DNA Structure and Functions
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Outline

• Genetic Material
• Transformation
• Viral Reproduction
• Bacteriophages
• Hershey and Chase
• DNA Structure
• Watson and Crick
• DNA Replication
• Prokaryotic versus Eukaryotic
• Replication Errors

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Genetic Material

• In 1931, bacteriologist Frederick Griffith
  performed an experiment with the bacterium
  Streptococcus pneumoniae and concluded that
  virulence passed from the dead strain to the
  living strain.
• Transformation
• Further research by Oswald Avery and others
  discovered that DNA is the transforming substance.

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Griffiths Transformation Experiment
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Reproduction of Viruses

• Bacteriophages are viruses that infect bacteria.
• Viruses consist of a protein coat (capsid)
  surrounding a nucleic acid core.
• Phage DNA contains genetic information necessary
  to produce new phages.

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Bacteria and Bacteriophages
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Reproduction of Viruses

• In 1952, Hershey and Chase used radioactive
  elements to differentially label the DNA core and
  protein capsid of a phage.
• Results indicated DNA, not the protein, enters
  the host, where viral reproduction takes place.

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Hershey and Chase
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Structure of DNA

• Nucleotide Data
• DNA contains
• Two with purine bases
• Adenine (A)
• Guanine (G)
• Two with pyrimidine bases
• Thymine (T)
• Cytosine (C)

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Chargaffs Rules

• The amount of A, T, G, and C in DNA varies from
  species to species.
• In each species, the amount of
• AT
• GC
• Complementary base pairing

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Base Sequence Variability

• It has been estimated the human chromosome
  contains, on average, 140 million base pairs.
• Number of possible nucleotide sequences
• 4140,000,000

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Watson and Crick Model

• James Watson and H. C. Crick constructed a model
  of DNA for which they received a Nobel Prize in
  1962.
• Double-helix model is similar to a twisted
  ladder.
• Sugar-phosphate backbones make up the sides.
• Hydrogen-bonded bases make up the rungs.

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Watson and Cricks DNA Model
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DNA Replication

• Each old strand of DNA serves as a template for a
  new strand.
• Semiconservative - One old strand is conserved in
  each daughter molecule.
• Steps
• Unwinding
• Complementary BasePairing
• Joining
• DNA polymerase

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Meselon and Stahls DNA Replication
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Prokaryotic Versus Eukaryotic Replication

• Prokaryotic Replication
• Bacteria have a single circular loop.
• Replication moves around the circular DNA
  molecule.
• May be able to divide as soon as once every 20
  minutes.

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Prokaryotic Versus Eukaryotic Replication

• Eukaryotic Replication
• DNA replication begins at numerous origins of
  replication along the length of the chromosome.
• Replication bubbles spread bi-directionally until
  they meet.
• Replication Fork

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Prokaryotic versus Eukaryotic Replication
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Replication Errors

• A genetic mutation is a permanent change in base
  sequences.
• Some mutations are due to errors in DNA
  replication, while others are due to to DNA
  damage.
• DNA repair enzymes are usually available to
  reverse most of these changes.
• Genetic variations are the raw material for
  evolutionary change.

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Review

• Genetic Material
• Transformation
• Viral Reproduction
• Bacteriophages
• Hershey and Chase
• DNA Structure
• Watson and Crick
• DNA Replication
• Prokaryotic versus Eukaryotic

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