Plasmider

Plasmider are small, extrachromosomal DNA molecules found in many bacteria and some archaea and eukaryotes. They are usually circular and double-stranded, and they replicate independently of the bacterial chromosome. Plasmider typically carry genes that confer selective advantages under certain conditions, such as antibiotic resistance, metabolic capabilities, or virulence factors, but they can also be cryptic with no obvious phenotype.

Most plasmider are present in multiple copies per cell, but copy number varies from one to hundreds. Their replication is governed by an origin of replication (ori) and regulatory systems that coordinate with the host cell. They are maintained through stable inheritance during cell division and can be transmitted horizontally between cells by conjugation, or via uptake and integration through transformation and transduction. Plasmider are classified into incompatibility groups, where related plasmids cannot coexist in the same host due to shared replication or maintenance mechanisms. Conjugative plasmider carry transfer genes (tra) enabling cell-to-cell transfer, while non-conjugative plasmider rely on other elements for movement.

Because plasmider can accumulate useful genes, they are central tools in molecular biology and biotechnology. They are widely used as cloning vectors and expression vectors to introduce and express foreign genes in bacteria. Typical features include an origin of replication, a multiple cloning site, and a selectable marker such as an antibiotic resistance gene. In nature, plasmider facilitate horizontal gene transfer, contributing to rapid adaptation and the spread of traits like antimicrobial resistance.