Replication

Microbial Genetics

Introduction to DNA Replication

  • Purpose of Replication:

    • Reproduction of genetic material.

    • Creation of more cells.

Key Components of Replication

  • Pyrimidines: Chemicals involved in DNA synthesis.

  • Semi conservative process . each new DNA molecule consists of one original parent strand and one newly synthesized daughter strand.

Step 1: Initiation of Replication

  • Location:

    • Occurs in the nucleoid region of the cell.

  • Process:

    • Helicase unwinds the DNA helix, creating a replication fork.

    • RNA Primase synthesizes an RNA primer that lays down RNA bases in a 5' to 3' direction.

    • This primer indicates to DNA Polymerase III where to start replication.

Step 2: Elongation Phase of Replication

  • Continuous Strand Elongation:

    • RNA Primase lays down one RNA primer at the replication fork.

    • DNA Polymerase III extends the primer in the 5' to 3' direction, continuously adding nucleotides.

    • Once the RNA primer is laid down, DNA Polymerase I removes the primers replacing them with DNA bases through exonuclease activity.

    • DNA Ligase seals any gaps between fragments of DNA that were formed during replication.

  • Lagging Strand Elongation:

    • For the lagging strand, the RNA primase lays down several RNA primers as the replication fork opens.

    • Each RNA primer is extended 5’ - 3’ away from the fork by DNA pol III making Okazaki fragments.

    6. Replacement of RNA Primers

    • Following the elongation phase, DNA pol I removes RNA primers and fills gaps w/ DNA via exonuclease activity

    7. Sealing of Gaps

  • DNA Ligase seals all remaining gaps, completing the DNA strand formation.

Additional Notes

  • Energy Source:

    • DNA triphosphate deoxyribonucleotides (dNTPs) provide the energy needed for DNA polymerization.

  • Helicase's Role:

    • Unwind the DNA to allow access for replication.

Summary of Key Enzymes

  • Helicase: Opens the DNA double helix.

  • RNA Primase: Lays down RNA primers for DNA synthesis.

  • DNA Polymerase I: Removes RNA primers and fills gaps with DNA.

  • DNA Polymerase III: Extends the leading strand.

  • DNA Ligase: Joins DNA fragments together by sealing nicks in the sugar-phosphate backbone.