DNA replication

• When does a cell copy DNA?

  • DNA needs to be copied before a cell divides or the chromosome number will half with each division
  • This takes place during S or Synthesis Phase of interphase
  • New cells will need identical DNA strands.
• • When in the life of a cell does DNA have to be copied?
  • Mitosis
  • Meiosis
• STEP 1: replication fork formation

  • Helicase enzyme (‘unzipping enzyme’) unwinds and separates the 2 DNA strands by breaking the weak hydrogen bonds between the nitrogenous bases.
  • 2 strands open forming a replication fork (y strand region)
  • New strands will grow at the forks
• STEP 2: DNA Priming

  • Primer must be added to each strand
  • The primer is synthesised by the enzyme Primase
  • Works as a starting position marker for replication.
• What is the role of DNA polymerase in DNA replication?

  DNA polymerase adds complementary bases to the template DNA strand.
• Why can DNA polymerase only add nucleotides to the 3' end of the DNA?

  Because DNA synthesis occurs in a 5' to 3' direction.
• Which strand of DNA has the 3' end exposed during replication?

  The leading strand has the 3' end exposed.
• How is the leading strand synthesized during DNA replication?

  The leading strand is synthesized as a single strand moving toward the opening replication fork.
• In which direction are new bases added during DNA synthesis?

  New bases are added in a 5' to 3' direction.
• How is the lagging strand synthesized during DNA replication?

  The lagging strand is synthesized discontinuously against the overall direction of replication.
• What are Okazaki fragments and how do they relate to the lagging strand?

  Okazaki fragments are short segments made on the lagging strand, adding bases in a 5' to 3' direction.
• From which direction is the lagging strand replicated?

  The lagging strand is replicated from the replication fork toward the origin.
• What is the function of the primer in DNA replication?

  The primer provides a starting point for DNA polymerase to begin synthesis.
• What is the significance of the replication fork in DNA synthesis?

  The replication fork is the area where the DNA double helix is unwound to allow for replication.
• STEP 4: termination and Proofreading

  • Exonuclease enzymes remove primers and proofreads and corrects any mistakes
  • Anything enzyme called ligase glues the Okasaki fragments together.
• How does the process of DNA replication ensure accuracy in base pairing?

  DNA polymerase checks for correct base pairing as it adds nucleotides to the growing strand.