Plasmids

Plasmids are small, circular, double-stranded DNA molecules that replicate independently of a cell’s chromosomal DNA. They are found mainly in bacteria and archaea, and occasionally in fungi and other organisms. Plasmids are extragenomic genetic elements because they maintain their own replication origins and can exist in multiple copies within a cell.

They commonly carry genes that confer advantageous traits, such as antibiotic resistance, metabolic capabilities, or virulence factors, though many plasmids are cryptic with no obvious effects. Copy number can range from one to hundreds per cell and is tightly controlled by the plasmid’s replication system. Partitioning and toxin-antitoxin systems help ensure inheritance during cell division.

Transfer between cells is possible through conjugation, transduction, or transformation. Conjugative plasmids carry genes for mating pair formation and transfer; non-conjugative plasmids may parasite transfer machinery or move with other elements. Horizontal transfer of plasmids plays a major role in the spread of antibiotic resistance and metabolic traits in microbial communities.

In research and biotechnology, plasmids are widely used as vectors to clone, express, or modify genes. Common features include an origin of replication, selectable marker genes, and a multi-cloning site. Plasmids such as pBR322 or pUC series have served as foundational tools, enabling protein production, gene function studies, and genetic engineering in bacteria and other hosts.

Plasmids are studied to understand replication control, host range, and evolutionary dynamics. They illustrate mobile genetic elements and how bacteria adapt to environmental pressures. The concept of plasmids underscores the fluidity of microbial genomes and the potential for rapid genetic change.