Plasmids Plasmids are molecules of DNA that are found in bacteria separate from the bacterial chromosome. Plasmids are replicated by the same machinery that replicates the bacterial chromosome. Some plasmids are copied at about the same rate as the chromosome, so a single cell is apt to have only a single copy of the plasmid. Other plasmids are copied at a high rate and a single cell may have 50 or more of them. Genes on plasmids with high numbers of copies are usually expressed at high levels.
Plasmids enter the bacterial cell with relative ease.
This occurs in nature and may account for the rapid spread of antibiotic resistance in hospitals and elsewhere. Plasmids can be deliberately introduced into bacteria in the laboratory transforming the cell with the incoming genes. The smaller the fragment, the farther it migrates in the gel. Sealed with DNA ligase , these molecules are functioning plasmids that are capable of conferring resistance to both ampicillin and kanamycin. They are molecules of recombinant DNA .