Plasmid

A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. These molecules are most commonly found in bacteria and archaea, but can also occur in eukaryotic organisms. Plasmids are generally circular or linear and range in size from a few thousand to hundreds of thousands of base pairs.

Plasmids often carry genes that benefit the survival of the organism, such as those conferring antibiotic resistance, encoding toxins, or enabling metabolic functions. These genes are not essential for the survival of the organism under normal conditions but can provide a selective advantage in specific environments.

Plasmids replicate independently of the host chromosome, typically using the host cell's replication machinery. The copy number of a plasmid within a cell can vary widely, depending on the plasmid and the host cell, ranging from a single copy to hundreds of copies.

Plasmids are important tools in molecular biology and genetic engineering. They are commonly used as vectors to clone and transfer genes between organisms. Modified plasmids can be introduced into bacteria to produce large quantities of specific proteins or to study gene function.