Plazmidok

Plazmidok are small, circular, double-stranded DNA molecules that replicate independently of the host genome. They occur mainly in bacteria and archaea, and less commonly in some eukaryotic microbes. Plazmidok are typically extrachromosomal and can exist in multiple copies per cell, depending on their origin of replication and regulatory elements.

Most plazmidok carry a replication origin (ori) and genes that can provide advantages to the host, such as antibiotic resistance, new metabolic capabilities, or virulence factors. Many also include partitioning systems to ensure stable inheritance during cell division and maintenance systems that help prevent loss.

Plazmidok are classified into incompatibility groups, defined by their replication control. Plazmidok from the same incompatibility group cannot stably coexist in the same cell, because they compete for the same replication machinery.

Some plazmidok are mobilizable or conjugative, containing transfer genes (tra) that enable transfer to other cells through conjugation. Others are non-mobilizable. The F plasmid in Escherichia coli is a well-known conjugative plasmid, while Col plasmids can encode bacteriocins that inhibit related strains.

In biotechnology, plazmidok are widely used as cloning and expression vectors. Typical plasmid vectors include an origin of replication, a selectable marker (such as an antibiotic resistance gene), a multiple cloning site for gene insertion, a promoter for expression, and often tags for protein purification. Copy number varies: high-copy plasmids yield many copies per cell, while low-copy plasmids reduce metabolic burden and can improve stability.

Natural and engineered plazmidok contribute to horizontal gene transfer, influencing microbial evolution, antibiotic resistance, and niche adaptation. Safety considerations address the potential spread of resistance genes and unintended ecological effects.