Thus, any mismatches made by Pol О± will be near the 5вЂІ ends of Okazaki fragments, in what can be called the О±-segment. Pol О±'s role in replication is to initiate Okazaki fragments by extending primers synthesized by its associated RNA primase. In contrast, the third nuclear replicase, Pol О±, lacks an intrinsic proofreading exonuclease (EXO) activity and is tenfold to 100-fold less accurate (Kunkel, 2009). These replicases are highly accurate, partly because their intrinsic 3вЂІ exonucleases can proofread the rare mismatches they generate. Tel: +1 6 E-mail: Дuring replication of the eukaryotic nuclear genome, most of the nascent leading and lagging strands are synthesized by DNA polymerase Оµ (Pol Оµ) and Pol Оґ, respectively. 6 Present address: Department of Molecular Physiology and Biophysics, Baylor College of Medicine, Houston, TX, USA.

5 Genome Integrity and Structural Biology Laboratory, National Institute of Environmental Health Sciences, NIH, DHHS, Research Triangle Park, NC, USA.4 Graduate Center for Toxicology, Markey Cancer Center, University of Kentucky College of Medicine, Lexington, KY, USA.3 Institute of Life Sciences, Jiangsu University, Zhen Jiang, Jiangsu, China.2 Departments of Radiation Biology and Molecular Medicine, City of Hope National Medical Center and Beckman Research Institute, Duarte, CA, USA.1 Colleges of Life Sciences and Agriculture and Biotechnology, Zhejiang University, Hangzhou, Zhejiang, China.