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

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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.

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.

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.

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.

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.

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