Plasmid copy number

Plasmids must regulate their copy number (average number of plasmid copies per cell) to ensure that they do not excessively burden the host or become lost during cell division. Plasmids may be either high copy number plasmids or low copy number plasmids; the regulation mechanisms between these two types are often significantly different. Biotechnology applications may involve engineering plasmids to allow a very high copy number. For example, pBR322 is a low copy number plasmid (~20 copies/cell) that has been adapted into very high copy number plasmid cloning vector (~1000 copies/cell).