Plasmids: structure, types, and lab use

Definition and core features of plasmids

A plasmid is a small, extrachromosomal DNA molecule found in cells that is physically separate from chromosomal DNA and can replicate independently. Plasmids are most commonly small circular, double-stranded DNA molecules in bacteria and archaea, though they can also occur in eukaryotes. They often carry genes that provide advantages under specific conditions—such as antibiotic resistance, virulence factors, genes for secondary metabolism, and functions that support survival in harsh environments (including heavy-metal resistance and bioremediation). Because they are additional to the essential genetic information on chromosomes, plasmids typically contain “special circumstance” genes rather than the core genes required for normal growth. Core plasmid features include the presence of a DNA origin of replication (making the plasmid a replicon) and a regulated copy number, which can range from one to hundreds or more copies per cell. Plasmids may be circular or, less commonly, linear (linear plasmids require specialized end-replication mechanisms). For stable inheritance during cell division, low-copy-number plasmids often encode partition systems (e.g., parABS/parMRC) that help distribute plasmid copies to daughter cells. Plasmids can also integrate into host chromosomes in some cases (episomes), and they vary widely in size (from <1 kbp mini-plasmids to >400 kbp, including megaplasmids). Functionally, plasmids can be classified by their roles (e.g., conjugative/fertility, resistance, virulence, degradative) and by how they are transferred between hosts (conjugative, mobilizable, or non-conjugative).

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Natural plasmid roles and carried gene functions

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