Circular DNA is found in:

Circular DNA is found in viruses (eg polyoma virus has double stranded & porcine virus has single stranded circular genome), prokaryotes (most bacterial DNA is circular), blue green algae as well as in eukaryotic cells like mitochondrial DNA or plasmid DNA and in chloroplast.A distinguishing characteristic of prokaryotes is their capacity to exchange small packets of genetic information carried on plasmids. Plasmid are small, specialized genetic element that are capable of replication within at least one cell line or exhibit broad hoast range and may carry sets of specialized genetic information (eg drug resistance) through a population. Plasmids were identified as small genetic elements carrying genes and capable of independent replication in prokaryotes (bacteria & yeast). Unlike viruses, plasmids are naked DNA and donot carry genes necessary to encase genetic material for transfer to new host. Sometimes plasmids may be found in eukaryotes like saccharomyces cerevisiae). In genetic engineering, plasmid is a small genetic (DNA) element that is separate from and can replicate independently of the chromosoml DNA. So introduction of DNA restriction fragment into a plasmid allows the fragment to be amplified many times.