EM

Table 9.3: DNA Replication Enzymes and Roles

Initiator protein

  • Binds to origin and separates strands of DNA to initiate replication.
  • This action initiates replication by opening the origin region so other replication proteins can load and start synthesis.

DNA helicase

  • Unwinds DNA at replication fork.
  • Creates and enlarges the replication fork by separating the two DNA strands, providing single-stranded templates for synthesis.

Single-strand-binding proteins (SSBs)

  • Attach to single-stranded DNA and prevent secondary structures from forming.
  • Stabilize the unwound DNA, keeping the template accessible for polymerases and preventing hairpins or reannealing.

DNA gyrase

  • Moves ahead of the replication fork, making and resealing breaks in the double-helix DNA to release the torque that builds up because of unwinding at the replication fork.
  • Also known as DNA topoisomerase II in many organisms; relieves supercoiling to prevent tangling and breakage of DNA.

DNA primase

  • Synthesizes a short RNA primer to provide a 3'‑OH group for the attachment of DNA nucleotides.
  • Primers are required for DNA polymerases to begin synthesis; typically one primer on the leading strand and multiple primers on the lagging strand.

DNA polymerase III

  • Elongates a new nucleotide strand from the 3' end; proofreads.
  • Polymerizes DNA in the 5'→3' direction on the growing strand and has high processivity.
  • Proofreading via a 3'→5' exonuclease activity to correct misincorporated nucleotides, increasing fidelity.

DNA polymerase I

  • Removes RNA primers and replaces them with DNA.
  • Exhibits 5'→3' exonuclease activity to remove RNA primers and 5'→3' polymerase activity to fill in with DNA; also participates in nick translation to replace primer regions.

DNA ligase

  • Joins Okazaki fragments by sealing breaks in the sugar–phosphate backbone of newly synthesized DNA.
  • Uses energy (ATP in many bacteria/eukaryotes; NAD+ in some bacteria) to form the phosphodiester bond and seal nicks between fragments.

Table 9.3

  • Recaps the roles of the initiator protein, DNA helicase, SSBs, DNA gyrase, primase, DNA polymerase III, DNA polymerase I, and DNA ligase.
  • Emphasizes the coordinated sequence of events at the replication fork and how each component contributes to establishing a complete, accurate replication process.