To evaluate the relative contributions or DNA polymerase α and DNA polymerase δ in chromosome replication during the S phase of the cell cycle, we have used the permeable cell system for replication as a functional assay. We carried out the analysis of DNA polymerases both in quiescent cells stimulated to proliferate and progress through the cell cycle (monolayers) and in actively growing cells separated into progressive stages of the cell cycle by centrifugal elutriation (suspension cultures). DNA polymerase α was measured by using the inhibitor butylphenyl dGTP at low concentrations. Using several inhibitors such as aphidicolin, ddTTP and butylphenyl dGTP, we found that DNA polymerase α and δ activity were coordinately increased during S phase and declined at the end. However, DNA polymerase δ was performing about 80% of the total replication and DNA polymerase α performed only 20%. This high ratio of DNA polymerase δ to DNA polymerase α replication activity was maintained throughout S phase in two entirely different experimental approaches.
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