Chapter 9 DNA: THE Genetic Material

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Chapter 9DNA THE Genetic Material
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Transformation

• Frederick Griffith, a bacteriologist, prepared a
  vaccine against pneumonia
• Vaccine a substance that is prepared from
  killed or weakened disease-causing agents,
  including certain bacteria
• To protect the body against future infections by
  the disease-causing agent

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Griffiths Experiments

• Griffith worked with 2 strains of S. pneumoniae
• Virulent (full of poison) able to cause disease
• 1st strain had a smooth capsule that protected
  the bacterium from bodys defense systems (S)
• 2nd strain lacked capsule and didnt cause
  disease (R)
• Mice injected with (S) strain died mice injected
  with (R) strain lived

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Griffiths Experiments

• Griffith injected mice with dead S bacteria
  mice lived
• Griffith injected mice with heat-killed S
  bacteria-mice still lived
• Meaning the capsule was not involved with killing
  the mice
• He mixed harmless live R bacteria with the
  harmless heat-killed S bacteria-mice died

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• Griffith had discovered what is now called
• Transformation- a change in genotype caused when
  cells take up foreign genetic material

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Averys Experiments

• In 1944, a series of experiments proved that
  transformation is stopped by DNA destroying
  enzymes.
• Almost 100 years after Mendels experiments,
  Oswald Avery co-workers demonstrated that DNA
  is the material responsible for transformation.

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Viral Genes and DNA

• In 1952, Hershey and Chase used T2 bacteriophage
  to prove that viruses infect bacteria.
• A bacteriophage, also referred as a phage, is a
  virus that infects bacteria.
• When phages infect bacteria they reproduce more
  viruses and burst to release more viruses.

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DNAs Role Revealed

• Alfred Hershey and Martha Chase performed The
  following experiment
• Step 1 grew E. coli labeled with radioactive
  sulfur (35S) protein coat. incorporated the
  sulfur
• Step 2 Labeled phages used to infect two separate
  batches of E. coli

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• Step 3 Used centrifuge to spin the tubes to
  separate the bacteria (heavy) from the viral
  parts (lighter).
• Concluded that the DNA of viruses is injected
  into the bacterial cells, while viral proteins
  coat was not.
• Injected DNA cause bacterial cells to produce
  more viruses.
• DNA is the heredity material in viruses.

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The Structure of DNA Section 2

• Watson Crick determined that a DNA molecule is
  a double helix two strands twisted around each
  other
• Nucleotides the subunits that make up DNA
• 3 parts a phosphate group, a 5-carbon sugar, and
  a nitrogen-containing base

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• DNA Base Pairing Rule
• Adenine (A) to Thymine (T) Guanine (G) to
  Cytosine (C)
• Adenine (A) and guanine (G) are classified as
  bulky purines two rings of carbon nitrogen
  atoms
• Thymine (T) and cytosine (C) are classified as
  the smaller pyrimidines single ring C N atoms

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Discovering DNAs Structure

• Chargaffs 1949 observations the amount of
  adenine always equaled the amount of thymine
  likewise the amount of guanine always equaled the
  amount of cytosine however the amount varied
  between different organisms.

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Wilkins Franklins Photographs

• X-ray diffraction to study the structures of
  molecules
• 1952 Wilkins Franklin developed high-quality
  X-ray diffraction photographs of strands of DNA
  which suggested that the DNA resembled a tightly
  coiled helix and was composed of two or three
  chains of nucleotides.

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Watson Cricks DNA Model

• 1953 Watson Crick to make the spiral
  staircase configuration of DNA.
• The model takes into account the information from
  Chargaff, Wilkins, Franklin along with their
  knowledge of chemical bonding.

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Pairing Between Bases

• Base-pairing rules cytosine pairs with guanine
  and adenine with thymine.
• These base pairing rules are supported by
  Chargaff's observations.
• The strictness of base-pairing results in 2
  strands containing complementary base pairs.

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The Replication of DNA section 3

• The complementary DNA structure serves as a
  template, or pattern, on which the other strand
  is built
• DNA replication the process of making a copy of
  DNA, which occurs during the (S) phase of the
  cell cycle

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• Step 1 The double helix needs to unwind before
  replication can begin
• Accomplished by enzymes called DNA helicases
  which open the double helix by breaking the
  hydrogen bonds between the two strands. The area
  where double helix separates is called
  replication fork.

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Step 2

• Additional proteins prevent the strands from
  assuming their double-helical shape
• Replication forks areas where the double helix
  separates
• Enzymes known as DNA polymerases add nucleotides
  to the exposed nitrogen bases, according to the
  base-pairing rules forming two double helixes

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• The process continues until DNA has been copied
  polymerases are signaled to detach
• Nucleotide sequences are identical in the two DNA
  molecules
• The enzyme helicase unzips the DNA
• DNA polymerase adds nucleotides to the exposed
  strands
• The end results are two identical strands, each
  strand with a parent strand and a daughter strand

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DNA Replication
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• Checking for errors
• errors sometime occur with wrong nucleotide added
  
• DNA polymerases proofreading role can
  backtrack and correct
• Proofreading reduces errors DNA replication
  about one error per 1 billion nucleotides

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Section 3
The Replication of DNA

• Replication doesnt begin at one end of DNA and
  end at the other.
• The circular molecules found in prokaryotes have
  two replication forks.
• Replication forks move away from one another
  until meeting at the other side of DNA circle.
• A Eukaryotic cell contains one single long strand
  of DNA.
• Each human chromosome is replicated in 100
  sections, each with its own starting point.
• Multiple replication forks working at same time.
  The entire chromosome replicates in 8 hours.

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Chapter 9.3