What is DNA replication? Describe the process of DNA replication in prokaryotes. (2024/15 Marks)

DNA Replication

• DNA replication is a biological process by which a cell duplicates its DNA, ensuring that each daughter cell receives an exact copy of the genetic material.
• It is essential for cell division (both mitotic and meiotic) and occurs in both prokaryotic and eukaryotic organisms.
• The process is highly regulated and involves the coordinated action of various enzymes and proteins.
• In prokaryotes, DNA replication occurs in the cytoplasm as they lack a membrane-bound nucleus.

DNA Replication in Prokaryotes

• Origin of Replication (OriC)
  
  • Prokaryotic DNA is typically circular.
  • The replication process begins at a specific region called the origin of replication (OriC) in E. coli (a model organism for prokaryotic DNA replication).
  • The OriC contains several DNA sequences recognized by initiator proteins.
• Unwinding of DNA (Helicase Activity)
  
  • DNA helicase (DnaB in E. coli) unwinds the double-stranded DNA at the replication fork, breaking hydrogen bonds between base pairs.
  • This results in two single-stranded DNA templates that can be used for replication.
• Formation of the Replication Bubble
  
  • As helicase continues to unwind the DNA, a replication bubble forms.
  • The bubble consists of two single-stranded DNA regions with a replication fork at each end.
• Stabilization of Single-Stranded DNA
  
  • Single-strand binding proteins (SSBs) bind to the single-stranded DNA to prevent it from re-annealing and protect it from nucleases.
• Primer Synthesis
  
  • Primase (DnaG in E. coli) synthesizes a short RNA primer that provides a 3’-OH group for DNA polymerase to begin adding nucleotides.
  • These primers are necessary because DNA polymerase can only add nucleotides to an existing strand.
• DNA Synthesis
  
  • DNA polymerase III (Pol III) is the main enzyme responsible for adding new nucleotides complementary to the template strand in a 5’ to 3’ direction.
  • On the leading strand, DNA synthesis occurs continuously as the replication fork progresses.
  • On the lagging strand, synthesis is discontinuous, resulting in the formation of Okazaki fragments.
• Okazaki Fragment Processing
  
  • The short segments of DNA (Okazaki fragments) on the lagging strand are synthesized in the 5’ to 3’ direction but are not continuous.
  • DNA polymerase I removes RNA primers and replaces them with DNA nucleotides.
  • DNA ligase joins the adjacent fragments by creating phosphodiester bonds, completing the synthesis on the lagging strand.
• Termination of Replication
  
  • DNA replication continues until the replication forks meet at the termination region, located opposite the OriC.
  • In E. coli, Tus proteins bind to the termination sequence, halting the progression of the replication fork.
  • The newly synthesized DNA strands separate, and the process is complete.

Conclusion

DNA replication is a complex and highly regulated process that ensures the accurate transmission of genetic information from one generation to the next. Understanding the intricacies of DNA replication in prokaryotes is essential for studying the fundamental principles of genetics and molecular biology.