Understanding Plasmids: Types, Functions, and Vectors

Definition and basic features of plasmids

A plasmid is a small, extrachromosomal DNA molecule found in cells that is physically separate from the chromosomal DNA and can replicate independently. They are most commonly small circular, double-stranded DNA molecules in bacteria and archaea, but plasmids can also occur in eukaryotes. Plasmids often carry genes that provide advantages under specific conditions—such as antibiotic resistance, virulence factors, genes for secondary metabolism, and functions for bioremediation—while chromosomes generally contain the essential genes needed for normal growth. Plasmids are considered replicons, meaning they contain an origin of replication (and related replication elements) that allows autonomous copying in a suitable host. They vary widely in size (from <1 kbp to over 400 kbp) and in copy number per cell (from one to thousands), with low-copy-number plasmids often using partition systems to ensure inheritance during cell division. Some plasmids can integrate into the host chromosome; in prokaryotes these integrative forms are sometimes called episomes. Plasmids can be transferred between bacteria—often across species—mainly via conjugation, and they are part of the mobilome; unlike viruses, they are typically “naked” DNA and do not encode a protective capsid for transfer (though some conjugative plasmids encode transfer machinery such as sex pili).

0/2 modes complete

Natural roles: antibiotic resistance and virulence

0/2 modes complete