Plasmids

 Plasmids are found in bacteria cells and are circular pieces of DNA that are seperate from the bacterial genome. They have the ability to replicate independently of the host cell, however their replication is dependent on particular enzymes being present in the host cell. Plasmids are often used as vectors to create transgenic bacteria, by inserting genes which aren't naturally present in the bacterial genome. Whilst most plasmids are circular, not all of them are. 

Hartl and Jones, 2009. Genetics: Analysis of Genes and Genomes. 7th Ed.