Plasmid is the class of the circular, extrachromosomal, autonomously replicating, DNA elements found in number of bacteria. Contain origins of the replication to ensure their maintenance. Some of the plasmids are capable of integrating into host genome. A number of artificially constructed plasmids are used as the cloning vectors or to change the characteristics of the bacteria. The common plasmids known are pBR322, pGEM, pUC18.