2.2 - DNA Replication

• What type of replication does DNA undergo?

  Semi-conservative replication
• What is the role of DNA helicase in replication?

  It breaks hydrogen bonds to unwind DNA
• What structure forms as DNA unwinds during replication?

  Replication fork
• What does primase synthesize during DNA replication?

  A small piece of RNA called primer
• What is the function of the primer in DNA replication?

  It marks the start point for new strand
• What does DNA polymerase do during replication?

  It catalyzes condensation reactions joining nucleotides
• In which direction is the leading strand synthesized?

  5' to 3' direction
• How is the lagging strand synthesized during DNA replication?

  In Okazaki fragments
• What initiates each Okazaki fragment on the lagging strand?

  A primer
• In which direction does DNA polymerase add bases on the lagging strand?

  S' to 3' direction
• What is the role of exonuclease in DNA replication?

  It removes primers from strands
• How does DNA polymerase fill gaps after primer removal?

  It fills gaps with DNA
• What is the function of DNA ligase in DNA replication?

  It seals fragments to form continuous strands
• What are the key steps in DNA replication?

  1. DNA helicase unwinds the DNA.
  2. Primase synthesizes RNA primers.
  3. DNA polymerase adds nucleotides.
  4. Leading strand is continuous; lagging strand in Okazaki fragments.
  5. Exonuclease removes primers.
  6. DNA polymerase fills gaps.
  7. DNA ligase seals fragments.
• why is DNA replication semi-conservative?

  each DNA molecule is made of one old strand and one new strand
• requirements for semi-conservative replication

  the four types of nucleotides each with their bases, must be present

  DNA polymerase

  both strands of DNA act as a template for attachment of nucleotides

  source of chemical energy to drive process
• why does DNA polymerase only work in one direction?

  its active site is only complementary to the 3'end of neem DNA Strand so can only add nucleotides at the 3'end so nem strand is made in 5' to 3' direction & DNA polymerase moves down template strand in 3' to 5' direction
• possible mechanisms for DNA replication in the past

  conservative model

  semi-conservative model

  dispersive model
• conservative model


  Original DNA Molecule remained intact ba separate daughter DNA copy was made from new molecules of deoxyribose, phosphate and organic bases
• dispersive model


  new DNA molecules had old & new DNA Mixed throughout them
• who provided evidence for semi-conservative replication?

  Meselson & Stahl
• what facts were Meselson & Stahl's experiment based on?

  all bases of DNA contain nitrogen

  nitrogen has 2 isotopes: 14N and 15N

  bacteria will incorporate nitrogen from their growing medium into any new DNA they make
• Meselson & Stahl's experiment

  E. Coli grown in nutrient solution containing 15N - heavy nitrogen became part of their DNA

  bacteria was transferred to a medium containg 14 N (light N) and allowed to replicate DNA

  sample of DNA was taken & centrifuged - lighter DNA will be at the top, heavier at the bottom

  DNA Settled in the middle so DNA Contained a mixture of light and heavy DNA so replicated semi-conservatively