A plasmid is a DNA molecule capable of replicating separately from a host cell’s main chromosome. Most familiar plasmids are circular, double-stranded molecules found in bacteria, although linear forms also exist. Plasmids frequently carry genes that affect survival under particular conditions rather than genes required for basic cellular functions. Their independent replication and capacity to carry inserted sequences make them important tools in recombinant DNA research. (genome.gov)
Structure and distribution
Plasmids vary greatly in size, genetic content, and organization. Small examples may contain little beyond the sequences needed for replication, whereas larger plasmids can carry extensive collections of accessory genes. Circularity is common but not defining: some bacterial linear plasmids have hairpin ends, while others have terminal proteins covalently attached to their DNA. These structures help address the challenges of maintaining and copying linear DNA ends. (pmc.ncbi.nlm.nih.gov)
Natural plasmids are not restricted to bacteria. They occur in archaea and in some eukaryotes, notably yeasts. The well-characterized 2-micron plasmid of Saccharomyces cerevisiae is a circular DNA element approximately 6.3 kilobases long that resides in the nucleus. Its genes and regulatory sequences largely support its own replication, segregation, and copy-number maintenance rather than providing an obvious accessory function for its host. (pmc.ncbi.nlm.nih.gov)
The distinction between plasmids and chromosomes is therefore not simply one of shape or size. Their replication systems, gene content, and relationship to the host’s essential genetic complement are also relevant. A plasmid is a genetic element, not an independently living organism: it depends on its host for resources and cellular machinery. (pmc.ncbi.nlm.nih.gov)
Replication and copy-number control
Independent replication means that plasmid copying is controlled separately from chromosome replication; it does not mean independence from the host. A plasmid contains an origin of replication, where DNA replication begins. Many plasmids encode an initiation protein, while host factors supply much of the machinery required to synthesize DNA. Circular bacterial plasmids use several replication mechanisms, including theta replication and rolling-circle replication. (pmc.ncbi.nlm.nih.gov)
Copy number is the number of copies of a particular plasmid maintained in a cell. Different replication systems support different characteristic copy numbers, ranging from very few copies to many copies. Regulatory mechanisms limit replication initiation, often through inhibitory RNA or interactions involving initiation proteins and repeated DNA sequences. Such controls balance continued plasmid inheritance against the burden of excessive replication. (pmc.ncbi.nlm.nih.gov)
Replication compatibility also limits which plasmids can coexist. Plasmid incompatibility is the inability of two plasmids to remain stably maintained together without appropriate selection. It commonly arises when their replication-control or partition systems interfere or fail to distinguish the two plasmid types. Plasmids are consequently classified into incompatibility groups, a scheme useful for studying natural plasmids and choosing combinations of laboratory constructs. (blog.addgene.org)
Inheritance and transfer
Plasmid persistence requires transmission to daughter cells during cell division. Many low-copy-number plasmids encode partition systems that position and segregate their DNA. These systems typically combine a specific DNA site with DNA-binding proteins and a motor protein. High-copy-number plasmids can benefit from having numerous copies available for distribution, although their inheritance is not necessarily explained by simple random dispersal alone. (pmc.ncbi.nlm.nih.gov)
Additional maintenance mechanisms include resolving linked plasmid copies into separate molecules and toxin–antitoxin systems. In some toxin–antitoxin systems, loss of the plasmid permits a longer-lasting toxin to act after its protective antitoxin disappears, inhibiting the growth or survival of plasmid-free descendants. These mechanisms promote retention without necessarily improving the host’s fitness. (pmc.ncbi.nlm.nih.gov)
Transfer between cells constitutes horizontal gene transfer. Conjugative plasmids encode the machinery needed for bacterial conjugation, which transfers DNA through direct contact between donor and recipient cells. Mobilizable plasmids possess transfer-related elements but depend on machinery supplied by another genetic element. Plasmids classified as nonmobilizable lack such conjugative capabilities, but may sometimes spread through uptake of environmental DNA or transfer mediated by bacteriophages. (pmc.ncbi.nlm.nih.gov)
Biological roles
Accessory genes carried by plasmids can alter a host’s interactions with its environment. Documented functions include antibiotic resistance, tolerance of toxic metals, virulence, and specialized metabolic capabilities. Transfer of a plasmid can therefore distribute several linked traits together, rather than requiring each recipient lineage to acquire them independently. (pmc.ncbi.nlm.nih.gov)
Plasmid carriage can also impose costs through DNA replication and gene expression. Whether a plasmid persists depends on the balance among these costs, benefits under particular environmental conditions, transfer opportunities, and inheritance mechanisms. Plasmids with no known beneficial accessory trait are often described as cryptic; this designation reflects incomplete functional knowledge, not proof that they have no biological effects. (pmc.ncbi.nlm.nih.gov)
Laboratory applications and analysis
Engineered plasmids serve as vectors for carrying selected DNA sequences into cells. A typical bacterial cloning vector contains a replication origin, a selectable marker, and an insertion region. Expression vectors additionally include regulatory elements, such as a promoter, that support gene expression in the intended host. A bacterial replication origin alone does not guarantee replication or expression in another organism. (genome.gov)
DNA fragments can be assembled into plasmids using restriction enzymes and DNA ligase, or alternative assembly methods. Researchers examine constructs through restriction analysis, gel electrophoresis, polymerase chain reaction, and DNA sequencing. Selection identifies cells carrying a marker, whereas sequence verification establishes whether the intended insert and surrounding construct are correct. (addgene.org)