In molecular biology and biochemistry, processivity is an enzyme's ability to catalyze "consecutive reactions without releasing its substrate".
When the polymerase advances along the DNA sequence after adding a nucleotide, the interactions with the minor groove dissociate but those with the phosphate backbone remain more stable, allowing rapid re-binding to the minor groove at the next nucleotide.
Their central pore is sufficiently large to admit the DNA strands and some surrounding water molecules, which allows the clamp to slide along the DNA without dissociating from it and without loosening the protein–protein interactions that maintain the toroid shape.
An example of such a DNA clamp is PCNA (proliferating cell nuclear antigen) found in S. cervesiae.
Both prokaryotes and eukaryotes must "trade" bound polymerases to make the transition from initiation to elongation.