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Encyclopedia of Genetics, Genomics, Proteomics and Informatics

pp 1672-1673

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Replication, Bidirectional

This is the mode of replication in bacteria as well as in the eukaryotic chromosome. Replication begins at an origin and proceeds in the opposite direction on both the old strands of the DNA double helix. The helicase subunits encoded by the xeroderma pigmentosum genes XPB and XpD of the transcription factor TFIIH unwinds the DNA in both directions. Electron microscope reveals a θ (theta) resembling structure of the circular DNA whereas in the linear eukaryotic DNA bubble-like structures are visible. In prokaryotes this replication is mediated by DNA polymerase III, and in eukaryotes a DNA polymerase α type enzyme. Termination of replication in E. coli requires 20 base long Ter elements and the associated protein Tus (termination utilization complex, Mr 36 K) (see Fig. R37). While replicating the template strand T7 RNA polymerase can by-pass up to 24 nucleotide gaps by making a copy of the deleted sequence using the corresponding non-template tract. DNA replication eukaryotes, DNA replication prokaryotes, θ replication, replication bubble, pol III, pol α, replication fork, xeroderma pigmentosum, transcription factors
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Figure R37.

Replication in E.coli. (Diagram after Kamada K et al 1996 Nature [Lond] 383:598)

The TerA, D and E stop the replication in an anticlockwise direction and TerC, B and F halt replication of the strand elongated clockwise. The Tus-Ter complex probably blocks the replication helicase. Similar mechanisms operate in most bacteria but replication fork arresting sites are also found in eukaryotes, including humans. (See Hiasa H, Marians KJ 1999 J Biol Chem 274:27244; Abdurasidova G et al 2000 Science 287:2023; Gerbi SA, Bielinsky AK 1997 Methods 13:271).

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