plasmíð

Plasmíð, or plasmid, is a small, circular DNA molecule that replicates independently of the bacterial chromosome. It is found primarily in bacteria and some archaea and often carries accessory genes that confer advantages under specific conditions, such as antibiotic resistance or the ability to metabolize unusual nutrients.

Plasmíðs are typically circular double-stranded DNA ranging from a few thousand to hundreds of thousands of base pairs. They replicate from their own origin of replication, or ori, and copy number can vary from a single copy to hundreds per cell. Replication and compatibility are influenced by plasmid-encoded factors and host cell machinery; plasmids are classified into incompatibility groups based on shared replication or partitioning systems.

Plasmíðs are categorized by their transfer capabilities and gene content. Conjugative plasmids carry tra genes and can transfer themselves to recipient cells through conjugation. Non-conjugative plasmids lack tra genes but may be mobilized by helper plasmids. Notable types include the F plasmid (fertility factor) in Escherichia coli, R plasmids carrying antibiotic resistance genes, Col plasmids encoding bacteriocins, and virulence plasmids that contribute to pathogenicity.

Conjugation is the primary horizontal transfer mechanism for plasmíðs, involving transfer of DNA through a pilus from donor to recipient cells. Plasmids can also disseminate genes through mobilization or, less commonly, be incorporated into the chromosome as episomes, facilitating genetic exchange within microbial communities.

In biotechnology, plasmíðs are widely used as cloning and expression vectors. They typically contain an origin of replication, selectable markers (often antibiotic resistance), and a multiple cloning site to insert foreign genes. While powerful, plasmíð-based methods require careful biosafety considerations due to concerns about antibiotic resistance and gene dissemination.