Covalently Closed Circular Molecule

General Information

 *  Because of their physical properties and small size, extra chromosomal circular DNAs are useful for studying the biology of genetic material. 
 * Recent studies have shown that the replication of different types of circular DNA, such as plasmids of Escherichia coli, the DNA of polyoma and SV40 viruses, and mitochondrial DNA is basically the same.
 * Non-replicating circular DNA is isolated from cells largely as covalently closed supercoiled molecules (form I DNA).
 * Because of its topological constraints, CCC DNA has a restricted affinity for intercalating dyes such asEthidium Bromide and propidium diiodide
 *   buoyant density in gradients of CsCl containing such dyes is consequently much greater than that of non-circular or nicked circular (form II) DNA species, which can unwind to accommodate maximal amounts of dye. 

Timmis, Kenneth, Felipe Cabello, and Stanley N. Cohen. "Covalently closed circular DNA molecules of low superhelix density as intermediate forms in plasmid replication."Nature Publishing Group.Nature 261, 512 - 516 .10 June 1976. http://www.nature.com/nature/journal/v261/n5560/abs/261512a0.html