Plasmids are small optional chromosomes carried in many bacterial cells. They are optional because the cells can survive without them but they may carry useful traits.  They have also been called extrachromosomal DNA to signify that they are DNA that isn’t part of the chromosome.  They can be easily extracted from cells and manipulated in the laboratory. By adding extra DNA to plasmids and reintroducing the enlarged plasmid back into living bacteria, it is possible to add new genes to the bacterium, as the newly introduced plasmid has the ability to multiply inside the bacteria and be inherited by all daughter bacteria that arise during cell division.