The commonly accepted model of replication depicts Pol ε stably anchored to CMG but shows Pol δ not physically tethered to the replisome and subsequently replaced for the synthesis of each Okazaki fragment ( Bell and Labib, 2016). On the lagging strand, Pol α-primase generates ~25-nt RNA-DNA primers that Pol δ extends to generate ~150-bp Okazaki fragments ( Bell and Labib, 2016). CMG unwinds DNA by translocating along one of the strands in a 3′ to 5′ direction while forming a complex with Pol ε ( Langston et al., 2014 Sun et al., 2015) to support highly processive synthesis of DNA on the leading strand. Key components include the 11-subunit CMG helicase and three different multi-subunit B-family DNA polymerases: the leading-strand Pol ε, lagging-strand Pol δ, and Pol α-primase ( Bell and Labib, 2016 Burgers and Kunkel, 2017). To robustly synthesize genomic DNA, the eukaryotic replisome requires a large number of interacting protein factors with different enzymatic activities.
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