DNA Replication in E. Coli

DNA Replication

Prior to cell division, genetic information stored in chromosomes must be copied via DNA replication, ensuring even distribution between two cells.

  • Understanding DNA replication largely comes from studies of E. Coli, bacteria abundant in the large intestine.

DNA Replication Process in E. Coli

  1. Initiation:

    • At the origin of replication, the two DNA strands separate, acting as templates for new strand synthesis.
    • This separation results in a "replication bubble."
    • The replication bubble expands in both directions, creating two "replication forks."
    • In human cells, multiple replication bubbles are formed simultaneously.

  2. Key Proteins at the Replication Fork:

    • DNA Helicase: Unwinds the double helix structure by disrupting hydrogen bonds between base pairs, creating a replication fork.
    • DNA Polymerase III: Builds new DNA strands by adding nucleotides to the 3' end of the existing strand. Requires a primer to initiate synthesis.
    • DNA Clamp Protein: A critical component associated with DNA polymerase III, that maintains the polymerase's association with the template strand, enhancing processivity.

  3. Semi-Conservative Replication:

    • Each resulting double helix consists of one original (parental) DNA strand and one newly synthesized DNA strand.

  4. Leading vs. Lagging Strand Synthesis:

    • Due to the antiparallel nature of DNA strands, replication occurs differently on each strand.
    • Leading Strand: Synthesized continuously in the 5' to 3' direction towards the replication fork.
    • Lagging Strand: Synthesized discontinuously in fragments (Okazaki fragments) also in the 5' to 3' direction, but overall moving away from the replication fork, requiring multiple primers.

  5. Leading Strand Synthesis in Detail:

    • DNA polymerase III adds DNA nucleotides one at a time to the 3' end of the leading strand.
    • Each new nucleotide pairs with its complementary nucleotide on the parental strand (A with T, G with C).

  6. Lagging Strand Synthesis - Complexity:

    • Primase: Synthesizes a short RNA primer, providing a starting point for DNA polymerase III.
    • DNA clamp protein attaches to DNA polymerase III which then adds deoxynucleotides to the 3' end of the RNA primer, extending the Okazaki fragment until it reaches the adjacent, previously synthesized fragment.
    • Polymerase detaches upon completion of the Okazaki fragment.

  7. Joining Okazaki Fragments:

    • DNA Polymerase I: Removes the RNA primers and replaces them with DNA nucleotides.
    • DNA Ligase: Catalyzes the formation of a phosphodiester bond to join the Okazaki fragments, creating a continuous strand.

  8. Termination:

    • Replication proceeds bidirectionally from the origin until the replication forks meet.
    • The result is two identical, double-stranded DNA molecules.

Relevance to Humans

  • While bacteria and humans are different, the fundamental mechanisms of DNA replication are conserved.
  • The process described in E. coli is similar to DNA replication in human cells.