Plasmida

Plasmida are small, circular, double-stranded DNA molecules that replicate independently of the host chromosome. They are extrachromosomal elements found primarily in bacteria, but also in some archaea, fungi, and plants. Plasmida typically range from a few thousand to several hundred thousand base pairs and can exist in multiple copies per cell (high-copy plasmida) or in a single or few copies (low-copy plasmida). Their replication origins and control systems determine copy number and compatibility with other plasmida.

Plasmida often carry accessory genes that confer selective advantages under certain conditions, such as antibiotic resistance, heavy-metal tolerance, or the ability to metabolize unusual compounds. They can also harbor virulence factors or metabolic genes. Many plasmida are readily transferred between cells via horizontal gene transfer methods, including conjugation, transformation, and transduction, contributing to genetic mobility among organisms.

Classification commonly uses incompatibility groups, defined by replication and maintenance systems that prevent coexistence of related plasmida in the same cell. They are valuable tools in biotechnology and molecular biology as cloning and expression vectors, selected for host range, copy number, and selectable markers. Plasmida are used to introduce genes, study gene function, or produce recombinant proteins.

Maintenance of plasmida depends on partitioning systems during cell division and, in some cases, post-segregational killing strategies to ensure persistence in a population. Plasmida can be lost if not maintained, particularly in the absence of selective pressure.