Prokaryotic DNA Replication- Enzymes, Steps and Significance

Prokaryotic DNA Replication- Enzymes, Steps and Significance

Prokaryotic DNA Replication- Enzymes, Steps and Significance

  • DNA replication is the process by which an organism duplicates its DNA into another copy that is passed on to daughter cells.
  • Replication occurs before a cell divides to ensure that both cells receive an exact copy of the parent’s genetic material. 
  • DNA replication uses a semi-conservative method that results in a double-stranded DNA with one parental strand and a new daughter strand.
  • Prokaryotic DNA replication is often studied in the model organism  coli, but all other prokaryotes show many similarities.

Prokaryotic DNA Replication

Features of Prokaryotic DNA Replication

  • Replication is bi-directional and originates at a single origin of replication (OriC).
  • Takes place in the cell cytoplasm.
  • Synthesis occurs only in the 5′to 3′direction.
  • Individual strands of DNA are manufactured in different directions, producing a leading and a lagging strand.
  • Lagging strands are created by the production of small DNA fragments called Okazaki fragments that are eventually joined together.

Enzymes of DNA Replication

  • Helicases: Unwind the DNA helix at the start of replication.
  • SSB proteins: Bind to the single strands of unwound DNA to prevent reformation of the DNA helix during replication.
  • Primase: Synthesizes the RNA primer needed for the initiation of DNA chain synthesis.
  • DNA Polymerase III (DNAP III): Elongates DNA strand by adding deoxyribonucleotides to the 3′end of the chain. Synthesis can only occur in the 5′to 3′direction because of DNAP III.
  • DNA Polymerase I (DNAP I): Replaces RNA primer with the appropriate deoxynucleotides.
  • DNA topoisomerase I: Relaxes the DNA helix during replication through creation of a nick in one of the DNA strands.
  • DNA topoisomerase II: Relieves the strain on the DNA helix during replication by forming supercoils in the helix through the creation of nicks in both strands of DNA.
  • DNA ligase: Forms a 3′-5′phosphodiester bond between adjacent fragments of DNA.

Steps of DNA Replication

Steps of DNA Replication

  • DNA replication begins at a specific spot on the DNA molecule called the origin of replication.
  • At the origin, enzymes unwind the double helix making its components accessible for replication. 
  • The helix is unwound by helicase to form a pair of replication forks.
  • The unwound helix is stabilized by SSB proteins and DNA topoisomerases.
  • Primase forms RNA primers (10 bases), which serve to initiate synthesis of both the leading and lagging strand.
  • The leading strand is synthesized continuously in the 5′to 3′ direction by DNAP III.
  • The lagging strand is synthesized discontinuously in the 5′to 3′ direction through the formation of Okazaki fragments.
  • DNAP I remove the RNA primers and replace the existing gap with the appropriate deoxynucleotides.
  • DNA ligase seals the breaks between the Okazaki fragments as well as around the primers to form continuous strands.

DNA REPAIR

Proofreading:

  • In bacteria, all three DNA polymerases (I, II and III) have the ability to proofread, using 3’ → 5’ exonuclease activity.
  • When an incorrect base pair is recognized, DNA polymerase reverses its direction by one base pair of DNA and excises the mismatched base.
  • Following base excision, the polymerase can re-insert the correct base and replication can continue.

Significance

  • DNA replication is a fundamental genetic process that is essential for cell growth and division. 
  • DNA replication involve the generation of a new molecule of nucleic acid, DNA, crucial for life.
  • DNA replication is important for properly regulating the growth and division of cells. 
  • It conserves the entire genome for the next generation.

References

  1. David Hames and Nigel Hooper (2005). Biochemistry. Third ed. Taylor & Francis Group: New York.
  2. Bailey, W. R., Scott, E. G., Finegold, S. M., & Baron, E. J. (1986). Bailey and Scott’s Diagnostic microbiology. St. Louis: Mosby.
  3. Madigan, M. T., Martinko, J. M., Bender, K. S., Buckley, D. H., & Stahl, D. A. (2015). Brock biology of microorganisms (Fourteenth edition.). Boston: Pearson.
  4. https://en.wikipedia.org/wiki/Proofreading_(biology)
  5. https://sciencing.com/comparing-contrasting-dna-replication-prokaryotes-eukaryotes-13739.html

Prokaryotic DNA Replication- Enzymes, Steps and Significance

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