Chapter 13: DNA

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Chapter 13 DNA
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I. Mystery of DNA Structure

• Griffith (1920s) Injected mice with deadly
• bacteria (rough colony) and they die.
• Inject mice with smooth bacteria and they
• live.
• Inject mice with heat-killed rough and
• they live.
• Mix heat-killed rough with healthy smooth,
  mice die.
• Conclusion Transforming factor smaller than
  cell.

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I. Mystery of DNA Structure

• B. Hershey/Chase (1950s)
• Radiolabeled proteins and DNA.
• Found viruses injected radiolabeled DNA,
• but no proteins.
• Conclusion DNA is transforming factor.

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Figure 16.2a The Hershey-Chase experiment phages
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virus particle labeled with 35S
virus particle labeled with 32P
bacterial cell (cutaway view)
label outside cell
label inside cell
Fig. 13.5, p. 217
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I. Mystery of DNA Structure

• C. Chargaff (1950s)
• Worked with DNA and found that the
• amount of adenine (A) equaled the
• amount of thymine (T) while the amount
• of cytosine (C ) equaled the amount of
• guanine (G).
• Conclusion Chargaffs base-pairing rule.

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I. Mystery of DNA Structure

• D. Franklin/Wilkins (1953) X-ray
• crystalography
• They froze DNA and ran x-rays through
• resulting crystal.
• Conclusion DNA tightly-wound helix
• (at least they provided the evidence
• for this conclusion).

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Figure 16.4 Rosalind Franklin and her X-ray
diffraction photo of DNA
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I. Mystery of DNA Structure

• Watson Crick (1953)
• Assemble all evidence and correctly
• interpret data to create double helix.
• Conclusion Current model of DNA
• (and Noble prize and fame and
• glory and chicks)

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II. Model of DNA

• Nucleotide Basic building block of DNA.
• 1 phosphate group, 1 deoxyribose
• sugar, 1 nitrogenous base.

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II. Model of DNA

• A. Nucleotide
• Sugar (Deoxyribose) Each carbon is
• numbered sequentially.

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2-nanometer diameter, overall
distance between each pair of bases 0.34
nanometer
each full twist of the DNA double helix 3.4
nanometers
Fig. 13.7, p. 219
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• Nucleotide
• 2. Phosphate Negative yet next to each other
  along DNA backbone, resulting in twisting.

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II. Model of DNA

• A. Nucleotide
• 3. Nitrogenous Bases
• Adenine and Guanine are Purines
• (double ring structures)
• Cytosine and Thymine are
• Pyrimadines (sing ring structures)

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phosphate group
ADENINE (A) base with a double-ring structure
GUANINE (G) base with a double-ring structure
sugar (ribose)
THYMINE (T) base with a single-ring structure
CYTOSINE (C) base with a single-ring structure
Fig. 13.6, p. 218
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II. Model of DNA

• B. Bonds within DNA Molecule
• 1. Phosphate to Sugar Covalent (note which
  carbon in sugar is attached to which phosphate).

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II. Model of DNA

• B. Bonds within DNA Molecule
• 2. Sugar to Nitrogenous Base Covalent.
• 3. N-base to N-base
• Hydrogen Bonds.

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II. Model of DNA

• C. Complete molecule is
• antiparallel (one side
• runs 3 to 5, other side
• runs upside down,
• or 5 to 3)

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For each, 1. How many base pairs?
2. How many nucleotides? 3.
How many sugars? 4. Circle a
hydrogen bond.
in-text, p. 219
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III. DNA Replication

• Replication Occurs in the nucleus process of
  DNA creating an exact replica of original strand.

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III. DNA Replication

• B. Semi-Conservative Nature of Replication
• The two daughter strands are half new and
  half original (order and molecules of parent
  strand are conserved ).
• Each half of original strand becomes a
  template for each new DNA molecule. The new
  strand is half old, half new.

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III. DNA Replication

• C. Steps of Replication
• 1. DNA uncoils and unzips (done by
• enzyme DNA helicase).
• 2. New nucleotides are added to exposed
• strand and added by DNA polymerase
• enzyme.
• 3. DNA ligase fills in gaps in new DNA
• strands.

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III. DNA Replication

• D. Okazawi Segements DNA is read from 3 to 5.
• 1. Leading strand reads 3 to 5 as DNA is
• unzipped.
• 2. Lag strand would have to wait for entire
• strand of DNA to unzip to begin rather
• it builds short segments, known as
• Okazawi segments (later stitched
• together with DNA ligase).

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Note 3 to 5 direction
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Figure 16.13 Synthesis of leading and lagging
strands during DNA replication
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Label Middle image 3 to 5
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Okazaki Segment Illustrated
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III. DNA Replication

• E. Replication Fork/Bubble
• As DNA unzips, it replication begins
• immediately. Replication simultanelously
• occurs at many sites along a strand of
• DNA.

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Figure 16.10 Origins of replication in eukaryotes