Genetic information not contained in the chromosome of bacteria or archaea is kept as circular double-stranded DNA molecules called plasmids (although some linear plasmids do exist). Plasmids contain only nonessential genes and replicate independent of the chromosome.
Some plasmids exist in many copies inside one cell and are called high-copy-number plasmids, whereas others are less numerous and are called low-copy-number plasmids. And of course, a cell can have many different plasmids at one time.
Although the enzymes involved in DNA replication of plasmids are the same as those used for the chromosome, some plasmids are copied in a different way than the chromosome. Circular chromosomes and some plasmids are copied in two directions at the same time starting at one point, called bidirectional replication. Other plasmids, however, are copied only in one direction, in a method called rolling circle replication.
One important feature of plasmids is that they can be transferred between bacteria. For the most part, this happens in two ways:
Bacteria die releasing their plasmids and other bacteria take them up. In order for this to happen, the second bacterium must naturally be able to take up DNA from its environment; it’s said to be naturally competent.
Plasmids, or other genetic material, are actively transferred from one bacterium to another by a process called conjugation. Only some plasmids have the genes necessary to induce the transfer of genetic material between bacterial cells via conjugation; they’re called conjugative plasmids. Conjugative plasmids can facilitate the transfer of themselves, other plasmids, and even chromosomal DNA between bacteria.
Aside from conjugative plasmids, the other main types of plasmids are
Resistance plasmids: Carry genes for resistance to antibiotics, heavy metals, and other cellular defenses.
Virulence plasmids: Carry virulence factors.
Colicin plasmids: Carry bacteriocins used to inhibit or kill other bacteria. Bacteriocins have a narrower range than antibiotics, so they’re specifically targeted to particular bacteria.
Plasmids are a useful tool for biotechnology.