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The remarkable processivity of cellular replicative DNA polymerases derive their tight grip to DNA from a ring-shaped protein that encircles DNA and tethers the polymerase to the chromosome. The crystal structures of prototypical 'sliding clamps' of prokaryotes (beta subunit) and eukaryotes (PCNA) are ring shaped proteins for encircling DNA. Although beta is a dimer and PCNA is a trimer, their structures are nearly superimposable. Even though they are not hexamers, the sliding clamps have a pseudo 6-fold symmetry resulting from three globular domains comprising each beta monomer and two domains comprising each PCNA monomer. These domains have the same chain fold and are nearly identical in three-dimensions. The amino acid sequences of 11 beta and 13 PCNA proteins from different organisms have been aligned and studied to gain further insight into the relation between the structure and function of these sliding clamps. Furthermore, a putative embryonic form of PCNA is the size of beta and thus may encircle DNA as a dimer like the prokaryotic clamps.