DNA: The Genetic Material

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DNA The Genetic Material

• Chapter 14

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The Genetic Material

• Frederick Griffith, 1928
• studied Streptococcus pneumoniae, a pathogenic
  bacterium causing pneumonia
• there are 2 strains of Streptococcus
• - S strain is virulent
• - R strain is nonvirulent
• Griffith infected mice with these strains hoping
  to understand the difference between the strains

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The Genetic Material

• Griffiths results
• - live S strain cells killed the mice
• - live R strain cells did not kill the mice
• - heat-killed S strain cells did not kill the
  mice
• - heat-killed S strain live R strain cells
  killed the mice

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The Genetic Material

• Griffiths conclusion
• - information specifying virulence passed from
  the dead S strain cells into the live R strain
  cells
• - Griffith called the transfer of this
  information transformation

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The Genetic Material

• Avery, MacLeod, McCarty, 1944
• repeated Griffiths experiment using purified
  cell extracts and discovered
• - removal of all protein from the transforming
  material did not destroy its ability to transform
  R strain cells
• - DNA-digesting enzymes destroyed all
  transforming ability
• - the transforming material is DNA

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The Genetic Material

• Hershey Chase, 1952
• - investigated bacteriophages viruses that
  infect bacteria
• - the bacteriophage was composed of only DNA and
  protein
• - they wanted to determine which of these
  molecules is the genetic material that is
  injected into the bacteria

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The Genetic Material

• - Bacteriophage DNA was labeled with radioactive
  phosphorus (32P)
• - Bacteriophage protein was labeled with
  radioactive sulfur (35S)
• - radioactive molecules were tracked
• - only the bacteriophage DNA (as indicated by the
  32P) entered the bacteria and was used to produce
  more bacteriophage
• - conclusion DNA is the genetic material

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DNA Structure

• DNA is a nucleic acid.
• The building blocks of DNA are nucleotides, each
  composed of
• a 5-carbon sugar called deoxyribose
• a phosphate group (PO4)
• a nitrogenous base
• adenine, thymine, cytosine, guanine

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DNA Structure

• The nucleotide structure consists of
• the nitrogenous base attached to the 1 carbon of
  deoxyribose
• the phosphate group attached to the 5 carbon of
  deoxyribose
• a free hydroxyl group (-OH) at the 3 carbon of
  deoxyribose

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DNA Structure

• Nucleotides are connected to each other to form a
  long chain
• phosphodiester bond bond between adjacent
  nucleotides
• formed between the phosphate group of one
  nucleotide and the 3 OH of the next nucleotide
• The chain of nucleotides has a 5 to 3
  orientation.

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DNA Structure

• Determining the 3-dimmensional structure of DNA
  involved the work of a few scientists
• Erwin Chargaff determined that
• amount of adenine amount of thymine
• amount of cytosine amount of guanine
• This is known as Chargaffs Rules

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DNA Structure

• Rosalind Franklin and Maurice Wilkins
• Franklin performed X-ray diffraction studies to
  identify the 3-D structure
• discovered that DNA is helical
• discovered that the molecule has a diameter of
  2nm and makes a complete turn of the helix every
  3.4 nm

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Fig. 16-6
(a) Rosalind Franklin
(b) Franklins X-ray diffraction
photograph of DNA
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Fig. 16-6a
(a) Rosalind Franklin
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Fig. 16-6b
(b) Franklins X-ray diffraction
photograph of DNA
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DNA Structure

• James Watson and Francis Crick, 1953
• deduced the structure of DNA using evidence from
  Chargaff, Franklin, and others
• proposed a double helix structure

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Fig. 16-1
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DNA Structure

• The double helix consists of
• 2 sugar-phosphate backbones
• nitrogenous bases toward the interior of the
  molecule
• bases form hydrogen bonds with complementary
  bases on the opposite sugar-phosphate backbone

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DNA Structure

• The two strands of nucleotides are antiparallel
  to each other
• one is oriented 5 to 3, the other 3 to 5
• The two strands wrap around each other to create
  the helical shape of the molecule.

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DNA Replication

• Matthew Meselson Franklin Stahl, 1958
• investigated the process of DNA replication
• considered 3 possible mechanisms
• conservative model
• semiconservative model
• dispersive model

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DNA Replication

• Bacterial cells were grown in a heavy isotope of
  nitrogen, 15N
• all the DNA incorporated 15N
• cells were switched to media containing lighter
  14N
• DNA was extracted from the cells at various time
  intervals

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DNA Replication

• The DNA from different time points was analyzed
  for ratio of 15N to 14N it contained
• After 1 round of DNA replication, the DNA
  consisted of a 14N-15N hybrid molecule
• After 2 rounds of replication, the DNA contained
  2 types of molecules
• half the DNA was 14N-15N hybrid
• half the DNA was composed of 14N

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DNA Replication

• Meselson and Stahl concluded that the mechanism
  of DNA replication is the semiconservative model.
• Each strand of DNA acts as a template for the
  synthesis of a new strand.

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DNA Replication

• DNA replication includes
• initiation replication begins at an origin of
  replication
• elongation new strands of DNA are synthesized
  by DNA polymerase
• termination replication is terminated
  differently in prokaryotes and eukaryotes

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Prokaryotic DNA Replication

• The chromosome of a prokaryote is a circular
  molecule of DNA.
• Replication begins at one origin of replication
  and proceeds in both directions around the
  chromosome.

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Prokaryotic DNA Replication

• The double helix is unwound by the enzyme
  helicase
• DNA polymerase III (pol III) is the main
  polymerase responsible for the majority of DNA
  synthesis
• DNA polymerase III adds nucleotides to the 3 end
  of the daughter strand of DNA

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Prokaryotic DNA Replication

• DNA replication is semidiscontinuous.
• pol III can only add nucleotides to the 3 end of
  the newly synthesized strand
• DNA strands are antiparallel to each other
• leading strand is synthesized continuously (in
  the same direction as the replication fork)
• lagging strand is synthesized discontinuously
  creating Okazaki fragments

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Fig. 16-9-1
A
T
C
G
T
A
T
A
C
G
(a) Parent molecule
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Fig. 16-9-2
A
T
T
A
C
G
G
C
A
T
A
T
T
T
A
A
C
C
G
G
(b) Separation of strands
(a) Parent molecule
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Fig. 16-9-3
A
A
T
T
A
T
T
A
C
C
G
G
G
C
G
C
A
T
A
A
T
A
T
T
T
T
A
T
T
A
A
A
C
C
G
C
C
G
G
G
(c) Daughter DNA molecules, each consisting of
one parental strand and one new strand
(b) Separation of strands
(a) Parent molecule
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Prokaryotic DNA Replication

• The enzymes for DNA replication are contained
  within the replisome.
• The replisome consists of
• the primosome - composed of primase and helicase
• 2 DNA polymerase III molecules
• The replication fork moves in 1 direction,
  synthesizing both strands simultaneously.

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Eukaryotic DNA Replication

• The larger size and complex packaging of
  eukaryotic chromosomes means they must be
  replicated from multiple origins of replication.
• The enzymes of eukaryotic DNA replication are
  more complex than those of prokaryotic cells.

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Eukaryotic DNA Replication

• Synthesizing the ends of the chromosomes is
  difficult because of the lack of a primer.
• With each round of DNA replication, the linear
  eukaryotic chromosome becomes shorter.

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Eukaryotic DNA Replication

• telomeres repeated DNA sequence on the ends of
  eukaryotic chromosomes
• produced by telomerase
• telomerase contains an RNA region that is used as
  a template so a DNA primer can be produced

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DNA Repair

• - DNA-damaging agents
• - repair mechanisms
• - specific vs. nonspecific mechanisms

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DNA Repair

• Mistakes during DNA replication can lead to
  changes in the DNA sequence and DNA damage.
• DNA can also be damaged by chemical or physical
  agents called mutagens.
• Repair mechanisms may be used to correct these
  problems.

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DNA Repair

• DNA repair mechanisms can be
• specific targeting a particular type of DNA
  damage
• photorepair of thymine dimers
• non-specific able to repair many different
  kinds of DNA damage
• excision repair to correct damaged or mismatched
  nitrogenous bases

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