DNA Structure and Replication

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• 13
• DNA Structure and Replication

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• I. Introduction and Review of Nucleic Acids and
  Genes
• A. Genes
• 1. HistoryKnown that chromosomes contain genetic
  information by 1920.
• 2. Genetic studies in the 1930s and 1940s
  indicate that DNA is the genetic information.
• 3. QuestionsHow does DNA specify a protein? How
  is it copied at each cell division?

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• The Watson-Crick Double Helix
• A. Identity of Watson and Crick
• B. Techniques they used to analyze structure of
  DNAx-ray crystallography with aid of Rosalind
  Franklin collaborating with Maurice Wilkins

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• The Watson-Crick Double Helix
• Nucleotides and Nucleic Acids
• Review three components of nucleotidesfive-carbon
  sugar, phosphate, and four types of
  nitrogen-containing bases.
• b) Joined together in covalent bonds between
  sugar and phosphate to make a chain

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• The Watson-Crick Double Helix
• Components of Model
• Nucleotides
• Nitrogen bases are laid out in specific and
  highly varied order, carrying code for protein
  synthesis.
• (animation)

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• Two chains wrapped around each other like a
  spiral staircase, with the sugar-phosphate
  covalent bonds making the sides of the staircase.

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• Two sides of staircase are held together by
  hydrogen bonds between the N- bases (A, C, G, and
  T) like steps of the staircase.
• Each base makes half a stair with the other
  always made up of its complement A T, and G
  C.

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• III. Replication
• 2. Each of the two original strands serves as a
  template (pattern) for construction of a new
  matching strand.

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• III. Replication
• B. Steps in DNA replication Figure 13.4
• 1. Double helix unwinds.
• 2. Nucleotides on each of two single strands are
  now available to form base pairs with their
  complement from a new free-floating nucleotide.
• 3. Both original strands can be copied, making
  two double helices from one original.
• Animation
• 4. The new helices are composed of half old
  (original) and half new nucleotides Figure 13

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DNA Zipper
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DNA Replication in Action
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New DNA Strands are Identical
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DNA is now copied so mitosis or meiosis can take
place.
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• 5. Replication is complex, but catalyzed by
  enzymes
• a) The addition and matching of new bases is
  catalyzed by DNA polymerase.
• b) DNA ligase permanently attaches short sections
  together to make one long chromosome.

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• III. Replication
• C. Importance of error-free copying
• 1. Error rate minimized by DNA polymerase
  proofreading.
• 2. Bacterial polymerase brings 10 million bases
  together incorrectly, but makes only 1/100,000
  mistakes.

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• Mutations
• A. Mutation permanent alteration in cells DNA
  base sequence Figure 13.6
• 1. Point mutationsslight change in chemical form
  of base, or incorrect base pairs
• 2. Chromosomal mutations Chapter 12
• B. Almost all cancers begin as a mutation that is
  passed along at replication.
• 1. In somatic cells (body cells)
• 2. Mutation rate is low, but after decades of
  accumulated mutations, cells can become
  malignant.

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• C. Heritable mutations occur in germ-line cells
  (cells that divide to make sperm and eggs).
• D. Heritable mutations also create genetic
  diversity.

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The End