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DNA Structure and Functions

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Title: A View of Life Author: Valued Gateway Client Last modified by: Media Created Date: 12/9/2002 2:36:15 AM Document presentation format: On-screen Show – PowerPoint PPT presentation

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Title: DNA Structure and Functions


1
DNA Structure and Functions
2
Outline
  • Genetic Material
  • Transformation
  • Viral Reproduction
  • Bacteriophages
  • Hershey and Chase
  • DNA Structure
  • Watson and Crick
  • DNA Replication
  • Prokaryotic versus Eukaryotic
  • Replication Errors

3
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.

4
Griffiths Transformation Experiment
5
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.

6
Bacteria and Bacteriophages
7
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.

8
Hershey and Chase
9
Structure of DNA
  • Nucleotide Data
  • DNA contains
  • Two with purine bases
  • Adenine (A)
  • Guanine (G)
  • Two with pyrimidine bases
  • Thymine (T)
  • Cytosine (C)

10
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

11
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

12
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.

13
Watson and Cricks DNA Model
14
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

15
Meselon and Stahls DNA Replication
16
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.

17
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

18
Prokaryotic versus Eukaryotic Replication
19
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.

20
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21
Review
  • Genetic Material
  • Transformation
  • Viral Reproduction
  • Bacteriophages
  • Hershey and Chase
  • DNA Structure
  • Watson and Crick
  • DNA Replication
  • Prokaryotic versus Eukaryotic

22
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