DNA replication

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DNA Replication

• By Pallavi Bohra

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What is DNA replication

• Process of producing two identical replicas from
  one original DNA molecule
• During preparation for cell division (S phase)
• Each strand of the original DNA molecule serves
  as a template 
•  Semiconservative replication concept

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Semiconservative model
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keynotes

• Helicase separates the DNA to form a replication
  fork at the origin of replication where DNA
  replication begins.
• Replication forks extend bi-directionally as
  replication continues.
• Okazaki fragments are formed on the lagging
  strand, while the leading strand is replicated
  continuously.
• DNA ligase seals the gaps between the Okazaki
  fragments.
• Primase synthesizes an RNA primer with a free
  3'-OH, which DNA polymerase III uses to
  synthesize the daughter strands.

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Prokaryotic Replication

• Inside cytoplasm
• Only one origin of replication 1000-2000nt
• Rich in AT sequences
• Two replication fork form in each DNA-bidirection
• Only one replicon

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Enzymes (Prokaryotic) involved..
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Enzymes involved
Enzymea Subunits Exnuclease activity 3-5- Exonuclease activity 5-3 Function
DNA poly I 1 Yes yes DNA repair and gap filling
DNA poly II - Yes No DNA repair
DNA poly III 10 Yes No Main replicating enzyme
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Bacteriophage replication Rolling circle

• Nick by rep protein (replication initiator
  protein) within ori.
• Rep protein binds to the 5 end of the nicked
  strand while the free 3-OH end is used as a
  primer for DNA synthesis (by host machinery, poly
  III)
• Other enzymes- helicase called PcrA
• Multiple copies of linear single stranded
•  DNA polymerase I removes the primer, replacing
  it with DNA, and DNA ligase

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Rolling circle replication
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Eukaryotic Replication
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