Working with Plasmids

Plasmid specifications

Plasmids vary widely in their copy number (see table Origin of replication and copy numbers of various plasmids and cosmids), depending on the origin of replication they contain (pMB1 or pSC101 for example) which determines whether they are under relaxed or stringent control; as well as the size of the plasmid and its associated insert. Some plasmids, such as the pUC series and derivatives, have mutations which allow them to reach very high copy numbers within the bacterial cell. Plasmids based on pBR322 and many cosmids are generally maintained at lower copy numbers. Very large plasmids are often maintained at very low copy numbers per cell.
Origin of replication and copy number of various plasmids and cosmids
DNA construct Origin of replication Copy number   Classification
pUC vectors pMB1* 500–700 High copy
pBluescript vectors ColE1 300–500 High copy
pGEM vectors pMB1* 300–400 High copy
pTZ vectors pMB1* >1000 High copy
pBR322 and derivatives pMB1* 15–20 Low copy
 pQE vectors ColE1 ~30 Low copy
pREP4 P15A ~30 Low copy
pACYC and derivatives P15A 10–12 Low copy
pSC101 and derivatives pSC101 ~5 Very low copy
SuperCos pMB1* 10–20 Low copy
ColE1 10–20 Low copy