Bacterial DNA clamp

"A DNA clamp, also known as a sliding clamp, is a protein fold that serves as a processivity-promoting factor in DNA replication."
-http://en.wikipedia.org/wiki/DNA_clamp
DNA clamps surround double-stranded DNA and act as a mobile tether for many partner proteins including replicative DNA polymerases.

This model is a surface representation of a crystal structure of the DNA clamp from E. coli (PDB entry 4K3S). Two identical subunits come together to make the clamp:
http://www.rcsb.org/pdb/explore/explore.do?structureId=4k3s

STL file created using VMD:
http://www.ks.uiuc.edu/Research/vmd/