Repair replication measurement

DNA single-strand break frequency was measured by alkaline elution [16] and sister chromatid exchanges (SCEs) were assayed by the method described by Wolff [32]. Hypoxanthine-guanine phosphoribosyl transferase (HPRT 6-thioguanine) and Na/K ATPase (ouabain) resistant mutants of CHO were determined by the methods described by Cleaver [7]. Repair replication after MMS treatment was measured in isopycnic gradients [8]. 

When base selection and proofreading are combined, DNA polymerase leaves behind one net error for every 106 to 108 bases added. Yet the measured accuracy of replication in E. coli is higher still. The additional accuracy is provided by a separate enzyme system that repairs the mismatched base pairs remaining after replication. We describe this mismatch repair, along with other DNA repair processes, in Section 25.2. 

Liver is the preferred tissue for UDS measurement because it is well-perfused and the main metabolic site for absorbed compounds (during first-pass of compounds administered by the oral and intraperitoneal routes). Moreover, it is a slow-dividing tissue, and only a small proportion of cells undergo replicative DNA synthesis. Therefore DNA synthesis in most liver cells is limited to DNA repair (Butterworth et al. 1987 Mirsalis and Butterworth 1980). 

DNA polymerase processivity is a measure of the average number of nucleotides added by the enzyme per one association/disassociation with the template. DNA polymerases associated with the DNA replication tend to be highly proces-sive, while those associated with the DNA repair tend to have low processivity. Because the binding of the polymerase to the template is the rate-limiting step in the DNA synthesis, the overall rate of the DNA repUcation during the S phase of the cell cycle is dependent on the processivity of the DNA polymerases performing the replication [11]. 

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