Chapter 9: DNA: The Molecule of Heredity

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Chapter 9DNA The Molecule of Heredity
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DNA The Early Years

• 1868 - Johann Frederich Miescher isolated DNA
  from the pus of open wounds and the sperm of a
  fish. He called the substance nuclein.
• It had the properties of an acid and it contained
  phosphorus.
• His discovery caused little excitement in the
  scientific community because at the time little
  was known about inheritance and it was thought
  that the hereditary material was protein.

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DNA The Early Years

• 1928 - while attempting to develop a vaccine for
  Streptococcus pneumoniae, Frederick Griffith
  found that a harmless strain of the bacteria
  could pick up information from a dead infectious
  strain and become infectious.

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Bacterial strain(s) injected into mouse
Results
Conclusions
R strain does not cause pneumonia.
Living R strain
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S strain causes pneumonia.
Living S strain
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Heat-killed S strain does not cause pneumonia.
Heat-killed S strain
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A substance from heat-killed S strain can
transform the harmless R strain into a deadly S
strain.
Living R strain, heat-killed S strain
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Bacterial strain(s) injected into mouse
Results
Conclusions
R strain does not cause pneumonia.
Living R strain
S strain causes pneumonia.
Living S strain
Heat-killed S strain does not cause pneumonia.
Heat-killed S strain
A substance from heat-killed S strain can
transform the harmless R strain into a deadly S
strain.
Living R strain, heat-killed S strain
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DNA The Early Years

• 1944 Oswald Avery and colleagues performed
  similar experiments in which they transformed
  harmless bacteria cells with extracts of dead
  pathogenic cells.
• They found that treating the extracts with DNA
  digesting enzymes blocked this transformation but
  treating with protein digesting enzymes did not
  block transformation.
• 1950s Using bacteriophages, Delbrück, Hershey,
  Chase, and Luria provided strong evidence that
  DNA and not protein is the molecule of
  inheritance.

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Bacteriophage

• Bacteriophage are virus particles that infect
  bacterial cells.

genetic material
bacterial cell wall
plasma membrane
viral coat
sheath
base plate
tail fiber
cytoplasm
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virus particle labeled with 35S
virus particle labeled with 32P
Hershey-Chase Experiments
bacterial cell
label inside cell
label outside cell
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DNA Structure Solved

• 1953 James Watson and Francis Crick solved the
  structure of DNA by studying the work of x-ray
  crystallographers Rosalind Franklin and Maurice
  Wilkins and the work of Erwin Chargaff.
• The x-ray data showed the molecule to be long
  skinny and spiral-like. (The Double Helix)
• Chargaffs data indicated that 1) the amount of
  pyrimidines always amount of purines and 2) the
  amount of G amount of C and the amount of A
  amount of T but the amount of AT is not
  necessarily the amount of GC.

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DNA X-Ray
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DNA Today

• DNA is composed of 4 types of nucleotides
• Nucleotides consist of a pentose sugar, a
  phosphate group, and a nitrogen containing base
• The 4 nucleotide bases of DNA are Adenine,
  Guanine, Cytosine, and Thymine
• Adenine and Guanine are Purines.
• Cytosine and Thymine are Pyrimidines.

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Nucleotide
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Pentose Sugar
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phosphate
DeoxyAdenylate
base adenine
sugar
phosphate
DeoxyGuanylate
sugar
base guanine
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DeoxyThymidylate
phosphate
base thymine

sugar
DeoxyCytidylate
phosphate
base cytosine

sugar
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Purines
phosphate group
Guanine (G) base with a double-ring structure
Adenine (A) base with a double-ring structure
sugar (ribose)
Pyrimidines
Thymine (T) base with a single-ring structure
Cytosine (C) base with a single-ring structure
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DNA Today

• These nucleotides link together to form strands
• Two strands then pair up to form a ladder-like
  structure
• In this ladder-like structure
• Adenine on one strand pairs with Thymine on the
  other strand
• Guanine on one strand pairs with Cytosine on the
  other strand
• This ladder-like structure then coils into a helix

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DNA Replication

• DNA replicates during the S-stage of the Cell
  Cycle.
• DNA replication is catalyzed by an enzyme called
  DNA Polymerase.
• A host of other enzymes and proteins are involved
  in DNA replication such as
• Helicases - separates the two strands
• Single-stranded binding protein
• Topoisomerases- unwind or create supercoils
• Ligases seal gaps in the new strand.
• Primase generates a small stretch of
  nucleotides (Primer)

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DNA Replication

• DNA replication occurs in a Semi-Conservative
  manner
• Each strand is used as a template to synthesize a
  new strand. Resulting in a new DNA molecule that
  contains a new strand and an old strand.

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free nucleotides
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Semi-Conservative DNA Replication
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DNA replication
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DNA Replication

• DNA Polymerase synthesizes the new strand in the
  5 to 3 direction and requires a primer in order
  to start stand extension.
• A primer is a short oligonucleotide that
  generates a duplex of DNA
• Because strand extension is carried out in a 5
  to 3 direction, One strand is synthesized
  continuously (leading strand). The other is
  synthesized in fragments (lagging strand).

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3 end
5 end
DNA Polymerase reads in the 3 to 5 direction.
_
C
G
H2C
O
H
H
DNA Polymerase synthesizes in the 5 to 3
direction.
H
H
H
H
H
H
H
H
O
_
H2C
A
DNA Polymerase needs a Primer to begin
synthesis.
T
H2C
_
O
H
H
H
H
5 end
OH
3 end
H
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replication forks
DNA helicase
DNA helicase
replication bubble
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DNA polymerase 1
continuous synthesis
discontinuous synthesis
DNA polymerase 2
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DNA polymerase 1 continues along parental DNA
strand
DNA polymerase 2 leaves
continuous synthesis
discontinuous synthesis
DNA polymerase 3
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DNA polymerase 3 leaves
DNA polymerase 4
DNA ligase joins daughter DNA strands
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replication forks
DNA helicase
DNA helicase
replication bubble
DNA polymerase 1
continuous synthesis
discontinuous synthesis
DNA polymerase 2
DNA polymerase 1 continues along parental DNA
strand
DNA polymerase 2 leaves
continuous synthesis
discontinuous synthesis
DNA polymerase 3
DNA polymerase 3 leaves
DNA polymerase 4
DNA ligase joins daughter DNA strands
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DNA Repair

• During replication mistakes can occur such as
  mismatched base pairs and mutated bases.
• In most cases these mistakes are fixed by special
  enzymes in the cell.
• However, if these enzymes are not operating
  properly, the mistakes could lead to genetic
  mutations that could then give rise to genetic
  disorders.

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Gene Mutations

• Nucleotide changes in the DNA sequence constitute
  gene mutations.
• Types of mutations
• Base-pair substitution
• Frameshift mutations (caused by insertions and
  deletions)
• Transposons - gene segments that spontaneously
  move from one location to another

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Summary

• DNA is composed of polymers of nucloetides.
• Nucleotides are composed of a pentose sugar
  (deoxyribose), a phosphate group, and a nitrogen
  containing base.
• DNA replication is catalyzed by DNA polymerase.
• DNA is replicated in a Semi-conservative manner
  with a leading and lagging strand.