DNA REPLICATION

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DNA REPLICATION
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• DNA gyrase the bacterial enzyme that relieves
  the tension produced by the unwinding of DNA
• DNA helicase enzyme that unwinds double-helical
  DNA by disrupting hydrogen bonds
• DNA polymerase I enzyme that removes RNA primers
  and replaces them with the appropriate
  deoxyribonucleotides
• DNA polymerase III enzyme responsible for
  synthesizing complementary strands of DNA
• DNA ligase enzyme that joins DNA fragments
  together by catalyzing the formation of a bond
  between the 3 hydroxyl and a 5 phosphate group
  on the sugar-phosphate backbone
• Primase enzyme that builds RNA primers

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• Leading strand
• Complementary strand is built toward replication
  fork
• Built continuously
• Primase needs to add only 1 RNA primer that DNA
  polymerase will use to build toward the
  replication fork

• Lagging strand
• Complementary strand is built away from
  replication fork
• Built discontinuously in small sections (Okazaki
  fragments)
• Primase continuously adds primers as replication
  fork travels along DNA molecule
• Gaps between Okazaki fragments are joined by DNA
  ligase

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• Replication fork
• Where the enzymes replicating a DNA molecule are
  bound to the untwisted, single-stranded DNA

• Replication bubble
• Formed when two replication forks are in close
  proximity to each other, resulting in a bubble of
  single-stranded DNA between them.

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Why eukaryotic DNA has multiple replication
origin sites

• Due to its size
• It would take too long for DNA replication to
  occur if replication started at one end of a DNA
  molecule and proceeded to the other end.
• The speed of DNA replication for the humans is
  about 50 nucleotides per second per replication
  fork. But the human Genome can be copied only in
  a few hours because because many replication
  forks take place at the same time (multiple
  initiation sites).

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DNA polymerase III, and DNA pol I act as quality
control by proof reading the new strands. When
mistakes occur either enzyme can function as an
exonuclease.
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