Plasmids are small, circular, double-stranded DNA (dsDNA) molecules that are separate from a chromosomal DNA of a cell. These extrachromosomal DNAs occur naturally in bacteria, yeast, and some higher eukaryotic cells. They exist in parasitic or symbiotic relationship with their host cell.

Generally, these plasmids have been engineered to optimize their use as vectors in DNA cloning. Most plasmid vectors contain a replication origin, a drug-resistance gene, and a region in which exogenous DNA fragments can be inserted as required for their use in DNA cloning.

They vary in size from few to several hundred kbp and in copy number. (A copy number is a measure of how many copies of plasmid may exist within a cell).

Genes on plasmids with high numbers of copies are usually expressed at a high level and they often encode proteins (Figure 2). Plasmids confer a selective advantage on the host by encoding their phenotypes, such as resistance to antibiotics and heavy metal, production of toxins or virulence factors, bacteriocin production, or catabolism of aromatic compounds.

Figure 2. A typical prokaryotic cell representing plasmid
Figure 2. A typical prokaryotic cell representing plasmid

Plasmids are categorized into two groups, conjugative and non-conjugative. They play a major role in bacterial gene transfer. Transmission of plasmid occurs either by the process of conjugation (conjugative plasmid) or mobilization (non-conjugative plasmid). These are grouped on the basis of their ability to coexist in the same bacterial host.

One of the most important features of plasmids is that they all possess an origin of replication (ori). This allows a plasmid to replicate within host cell independently of host-cell replication. The smaller plasmids make use of the host cell’s own DNA replicative enzymes in order to make their own copies, whereas some of the larger ones carry genes that code for special enzymes that are specific for plasmid replication. As several different kinds of plasmid are found in a single cell, they must be compatible with each other to coexist, which in turn prevent their loss from the cell.