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. Whilst most plasmids are circular, not all of them are [1][2].
Plasmids are often used as vectors to create transgenic bacteria, by inserting genes which are not naturally present in the bacterial genome. Genes are inserted into the vector by cutting it at specific restriction sites with enzymes called restriction endonuclease.