Bacterial Replication Fork: Synthesis of Lagging Strand


How can an antiparallel DNA (deoxyribonucleic acid) strand be duplicated by a DNA polymerase that synthesises DNA in only one direction? This paradox of DNA synthesis on the lagging strand was dissolved by discovery of Okazaki fragments. The major components of the bacterial replication fork include replicative helicase, primase and DNA polymerase. The loading of replicative helicase, DnaB, is the most critical step for assembly of a primosome, a protein complex responsible for duplex unwinding and primer RNA (ribonucleic acid) synthesis at the replication fork. DNA polymerase may be an asymmetric dimer, each of which may concurrently synthesise leading or lagging strand. Several different modes of primosome assembly have been identified in bacteria. At oriC (origin of chromosome), DnaA‐dependent primosome is assembled for initiation of a round of DNA replication, whereas PriA‐dependent primosome is assembled at stalled replication forks to facilitate replication restart.

Key Concepts:

  • Initiation of DNA replication: DNA replication is initiated by the initiator protein, which specifically recognises and binds to the origin sequence and recruits other primosome components including a DNA helicase.

  • Leading and lagging strands: Leading strand is the one in which the direction of DNA chain elongation and overall fork movement is the same and lagging strand is the one in which they are opposite.

  • Replicative helicase: An enzyme which catalyses continuous unwinding of the parental duplex DNA at the replication fork.

  • Replication fork: The site of DNA replication where two replicating single‐stranded DNA separates.

  • Primer RNA: A short stretch of RNA, the 3′‐terminus of which is utilised by DNA polymerases for DNA elongation.

  • Primosome: A name given to the protein complex capable of duplex DNA unwinding and primer RNA synthesis at the replication fork.

  • Stalled replication fork: A replication fork the movement of which is blocked by internal and external ‘replication stress’ including DNA damages and depletion of nucleotide precursors.

  • Replication restart: The process of reassembly of primosome at a stalled replication fork to resume DNA chain elongation.

Keywords: replication fork; Okazaki fragment; DNA polymerases; primer RNA; replication restart

Figure 1.

The end/polarity problem of a replication fork and Okazaki fragment model. (a) Unidirectional elongation of DNA chains in only 5′ to 3′ direction by DNA polymerases inevitably generates an unreplicated segment on one strand of template DNA at the replication fork (shown by dotted lines), as a replication fork propagates. (b) Reiji Okazaki proposed that the lagging strand (upper strand in this figure) is synthesised discontinuously through joining of small pieces of nascent DNA (Okazaki fragments). Red arrowed lines and orange wavy lines represent nascent DNA chains and primer RNAs, respectively.

Figure 2.

A model for the bacterial replication fork structure. The model shows concurrent replication of both strands by asymmetric twin DNA polymerases with a looped lagging strand DNA template. DnaB, located on the lagging strand template, unwinds duplex DNA and primase, in association with DnaB, generates primer RNAs for synthesis of multiple Okazaki fragments. SSB (single‐stranded DNA‐binding protein) protects the exposed single‐stranded DNA and facilitates the action of DNA polymerase. Swiverase (DNA topoisomerase) eliminates the positive supercoiling which would accumulate in the unreplicated duplex segment, as the replication fork progresses.



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Tanaka, Taku, and Masai, Hisao(Apr 2010) Bacterial Replication Fork: Synthesis of Lagging Strand. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001049.pub2]