DNA Replication

Eukaryotic DNA replication is more complex than the prokaryotic process but uses many enzymes analogous to those listed below. In both prokaryotes and eukaryotes, DNA replication is semiconservative, involves both continuous and discontinuous (Okazaki fragment) synthesis, and occurs in the 5′ → 3′ direction.

  • Origin of replication
    • Particular consensus sequence of base pairs in genome where DNA replication begins.
    • May be single (prokaryotes) or multiple (eukaryotes).
    • AT-rich sequences (such as TATA box regions) are found in promoters and origins of replication.
  • Replication fork
    • Y-shaped region along DNA template where leading and lagging strands are synthesized.
  • Helicase
    • Unwinds DNA template at replication fork.
    • Helicase Halves DNA.
  • Single-stranded binding proteins
    • Prevent strands from reannealing.
  • DNA topoisomerases
    • Create a single- or double-stranded break in the helix to add or remove supercoils.
    • In eukaryotes: irinotecan/topotecan inhibit topoisomerase (TOP) I, etoposide/teniposide inhibit TOP II.
    • In prokaryotes: fluoroquinolones inhibit TOP II (DNA gyrase) and TOP IV.
  • Primase
    • Makes an RNA primer on which DNA polymerase III can initiate replication.
  • DNA polymerase III
    • Prokaryotes only. Elongates leading strand by adding deoxynucleotides to the 3′ end.
    • Elongates lagging strand until it reaches primer of preceding fragment.
    • 3′ → 5′ exonuclease activity “proofreads” each added nucleotide.
    • DNA polymerase III
      • Synthesis:  5′ → 3′ polymerase
      • Proofreading: 3′ → 5′ exonuclease.
    • Drugs blocking DNA replication often have a modified 3′ OH, thereby preventing addition of the next nucleotide (“chain termination”).
  • DNA polymerase I
    • Prokaryotic only.
    • Degrades RNA primer; replaces it with DNA.
    • Same functions as DNA polymerase III but also excises RNA primer with 5′ → 3′ exonuclease.
      • Synthesis: 5′ → 3′ polymerase
      • Proofreading: 5′ → 3′ exonuclease, 3′ → 5′ exonuclease.
  • DNA ligase
    • Catalyzes the formation of a phosphodiester bond within a strand of double-stranded DNA.
    • Joins Okazaki fragments.
    • Ligase Links DNA.
  • Telomerase
    • Eukaryotes only.
    • A reverse transcriptase (RNA-dependent DNA polymerase) that adds DNA (TTAGGG) to 3′ ends of chromosomes to avoid loss of genetic material with every duplication.
    • Often dysregulated in cancer cells, allowing unlimited replication.
    • Telomerase TAGs for Greatness and Glory.

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