Plasmide

A plasmide, or plasmid, is a small, circular, double-stranded DNA molecule that replicates independently of the chromosomal DNA in a cell. Plasmides are most commonly found in bacteria and archaea, though some eukaryotic organisms also harbor plasmids. They are considered extrachromosomal genetic elements and typically carry one to several genes that confer advantageous traits but are not essential for basic survival under standard conditions.

Plasmide genomes include an origin of replication (ori) that enables autonomous replication and often a partitioning system to ensure stable maintenance during cell division. Copy number can vary widely, from only a few copies per cell to hundreds. Plasmides can transfer between cells by processes such as conjugation, transformation, or transduction; conjugative plasmids carry transfer genes and can mediate horizontal gene transfer.

Many plasmides harbor accessory genes, including antibiotic resistance determinants, metabolic enzymes, or virulence factors. Some plasmids, such as Col plasmids, encode toxins to suppress competitors, while others serve as fertility factors (F plasmids) that increase the likelihood of plasmid spread through a population. In molecular biology, plasmids are routinely engineered as cloning and expression vectors; common laboratory plasmids carry multiple cloning sites and selectable markers, such as antibiotic resistance genes, and are used to propagate, express, and purify genes and proteins.

Classification of plasmids into incompatibility groups (Inc) helps explain their maintenance and compatibility in a host. Plasmid research informs our understanding of evolution, ecology, and the spread of traits like antibiotic resistance, and it raises biosafety and ethical considerations regarding genetic exchange.