“…The replication clamps are ring-shaped complexes that encircle dsDNA and slide freely on the duplex, conferring processivity to their respective replicative DNA polymerase ( Kunkel and Burgers, 2014 ; Kunkel and Burgers, 2017 ; Nick McElhinny et al, 2008 ; ODonnell and Kurth, 2013 ; Pursell et al, 2007 ; Yao and O’Donnell, 2021 ). In eukaryotes, PCNA (Proliferating Cell Nuclear Antigen) functions with replicative polymerases Pol δ on the lagging strand and Pol ε on the leading stand ( Kunkel and Burgers, 2014 ; Kunkel and Burgers, 2017 ; Nick McElhinny et al, 2008 ; ODonnell and Kurth, 2013 ; Pursell et al, 2007 ; Yao and O’Donnell, 2021 ). Structural studies showed that, the E. coli β-clamp is a homodimer, while the T4 phage clamp and archaeal and eukaryotic PCNA clamps are homotrimers, all of which display six domains having similar chain folds and remarkable pseudo sixfold symmetric architecture with an obvious ancestor in evolution ( Gulbis et al, 1996 ; Kong et al, 1992 ; Krishna et al, 1994 ; Matsumiya et al, 2001 ; Yao and O’Donnell, 2016 ).…”