[9] The beta chain of bacterial DNA polymerase III is composed of three topologically equivalent domains (N-terminal, central, and C-terminal).

Two beta chain molecules are tightly associated to form a closed ring encircling duplex DNA.

[10] The sliding clamp in eukaryotes is assembled from a specific subunit of DNA polymerase delta called the proliferating cell nuclear antigen (PCNA).

Three PCNA molecules are tightly associated to form a closed ring encircling duplex DNA.

[12][13] In eukaryotes, a homologous, heterotrimeric "9-1-1 clamp" made up of RAD9-RAD1-HUS1 (911) is responsible for DNA damage checkpoint control.

This PCNA ring works with PolD, the single eukaryotic-like DNA polymerase in archaea responsible for multiple functions from replication to repair.

The giant virus genus Chlorovirus, with PBCV-1 as a representative, carries in its genome two PCNA genes (Q84513, O41056) and a eukaryotic-type DNA polymerase.

Each domain consists of two alpha helices and two beta sheets – the fold is duplicated and has internal pseudo two-fold symmetry.

The two-domain protein does, however, associate with the viral DNA polymerase and also acts to increase processivity.