Replication, Bidirectional

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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