DNA STRUCTURE AND REPLICATION

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DNA STRUCTURE AND REPLICATION

• Chapter 12

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• A. DNA Structure
• DNA is a nucleic acid composed of nucleotide
  monomers.
• DNA nucleotide consists of
• one phosphate group
• one deoxyribose sugar (5 carbon sugar)
• one nitrogenous base (G, A, C or T)

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• DNA is a double-stranded helix (Watson Crick
  1953).

• Sides of ladder make up sugar-phosphate
  backbone.
• Rungs of ladder composed of base pairs joined by
  hydrogen bonds.

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• Pyrimidines (T C) form hydrogen bonds with
  purines (A G).

• Thymine pairs with Adenine, forming 2 hydrogen
  bonds.
• Cytosine pairs with Guanine, forming 3 hydrogen
  bonds.

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• DNA strands are antiparallel.

Numbering of strands is based on position of
deoxyribose sugars.
5 to 3 strand 3 to 5 strand
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• DNA is highly condensed.

• DNA is wrapped tightly around proteins folded.
• DNA must unwind for replication to occur.

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• B. DNA Replication
• Process by which DNA is duplicated.
• occurs during the S phase of Interphase
• is semiconservative (Meselson Stahl)

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• Overview of DNA Replication

?Unreplicated DNA. ?Strands unzip at several
points creating replication forks. ?Each strand
serves as template for complementary nucleotides
to H-bond. ?New nucleotides of each daughter
strand are linked.
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• Steps in DNA Replication

?Helicase breaks hydrogen bonds. ?Binding
proteins stabilize strands prevent them from
rejoining. ?Primase makes an RNA primer.
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?Free nucleotides move in H-bond DNA
polymerase links nucleotides to each other
starting at primer working in the 5 to 3
direction. ?DNA polymerase proofreads new
strand (replaces incorrect bases).
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?DNA replication is continuous on one
strand. ?DNA replication is discontinuous on
other strand, producing Okazaki fragments.
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Ligase
?Repair enzymes remove RNA primers Ligase
connects Okazaki fragments.
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Determine the base sequence of daughter DNA
replicated from the following parental DNA strand.
parental DNA C T A G G T A C
T daughter DNA G A T C C A T G A
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• C. DNA Repair
• UV radiation damages DNA by causing thymine
  dimers to form.

DNA damage can be repaired by photoreactivation
or excision repair.
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• 1. Photoreactivation photolyase (enzyme) uses
  light energy to split dimer.

2. Excision repair - repair enzyme cuts out
damaged area DNA polymerase inserts replacement
sequence ligase seals backbone.
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• 3. Mismatch repair - enzymes proofread newly
  replicated DNA for base mispairing correct the
  error.
• Faulty DNA repair results in chromosome breaks
  an increased susceptibility to cancer.
• Ex. Xeroderma pigmentosum