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Short answer
The origin of replication of a plasmid, usually written ori, is the DNA sequence where replication begins. It recruits the host’s replication machinery, and it is the main reason a plasmid holds a given number of copies per cell and cannot sit alongside plasmids that share the same origin.
Definition and scope: ori versus replicon
Addgene’s Plasmids 101 series defines the ori as the place where DNA replication begins, the sequence that lets a plasmid initiate replication by recruiting replication machinery proteins. It also states that the origin controls the host range and the copy number of the plasmid. The replicon is wider than the ori: it is the origin of replication plus all of its control elements.
The distinction matters when reading a vector description. “pUC ori” names the origin family, while the replicon behaviour (how tightly copy number is regulated) depends on the control elements around it. Everything below concerns origins in bacterial plasmids, the case Addgene’s series covers. The term “plasmid ori” in a vector name or map feature list refers to this same sequence.
For the wider picture of what a plasmid and a vector are, see plasmid vector; for how ori sits among the other features on a map, see how to read a plasmid map.
How copy number and compatibility work
Copy number
According to Addgene, a plasmid’s copy number reflects the balance between positive and negative regulation of its replication, and it can be changed by mutations in the replicon. Their worked example: the pMB1 ori maintains about 20 copies per cell, while pUC, which differs from it by only two mutations, produces as many as 700 copies per cell. Addgene also notes that actual copy number varies, so any figure is an expected range rather than a guarantee.
Relaxed and stringent control
Addgene describes replication control as “relaxed” or “stringent” and gives a classification for each common vector in its table (below). Stringently controlled origins in that table (pR6K and pSC101) are among the lower-copy entries.
Incompatibility
Generally, plasmids with the same ori are incompatible: they compete for the same replication machinery, which creates an unstable and unpredictable environment in the cell. Addgene’s rule is that plasmids from the same incompatibility group should not be co-transformed. It also notes that the incompatibility group labels in its table are an arbitrary designation, not a universal standard.
Origins and copy numbers in common vectors
Every row below comes from the table in Addgene’s “Plasmids 101: Origin of Replication” article (checked 2026-10-29). Copy numbers are approximate per-cell figures, and Addgene notes that actual copy number varies.
| Common vector | Copy number per cell | ori | Incompatibility group | Control |
|---|---|---|---|---|
| pUC | ~500-700 | pMB1 (derivative) | A | Relaxed |
| pBluescript | ~300-500 | ColE1 (derivative) and F1 | A | Relaxed |
| pGEM | ~300-500 | pUC and F1 | A | Relaxed |
| pCDF | ~20-40 | CloDF13 (CDF) | D | Relaxed |
| pBR322 | ~15-20 | pMB1 | A | Relaxed |
| pET | ~15-20 | pBR322 | A | Relaxed |
| pGEX | ~15-20 | pBR322 | A | Relaxed |
| pColE1 | ~15-20 | ColE1 | A | Relaxed |
| pR6K | ~15-20 | R6K (requires the pir gene for replication) | C | Stringent |
| pACYC | ~10 | p15A | B | Relaxed |
| pSC101 | ~5 | pSC101 | C | Stringent |
The F1 listed for pBluescript and pGEM is a phage-derived origin that allows single-stranded DNA to be replicated and packaged into phage particles, so it serves a different purpose from the plasmid’s main ori.
Where it matters for labs: a checklist for reading a vector description
Because the table above ties each origin to a copy number, an incompatibility group and a host requirement, a vector datasheet or map can be checked against four points taken from Addgene’s guidance:
- Is the ori named? The origin sets the replication behaviour, so it should be identifiable in the feature list.
- What copy number does it imply? Compare the named ori with the table: a pUC-derived origin sits in the hundreds of copies per cell, a pSC101 origin near five.
- Is it in the same incompatibility group as another plasmid in the experiment? pET and pUC are both group A, so by Addgene’s rule they should not be co-transformed. pACYC is listed in group B, a different group from pET in the table, which is the kind of distinction the groups are meant to show.
- Does the origin have a host requirement? R6K needs the pir gene to replicate.
Addgene adds that most E. coli strains can propagate plasmids, though endA-negative strains are best for high plasmid yields. For strain handling, see competent cells, and for the wider cloning workflow, see gene cloning.
Other factors that affect copy number
Addgene stresses that the sequence and regulation of the ori dramatically affect copy number, but other factors contribute too. It lists the insert (bacteria tend to keep fewer copies of plasmids carrying large inserts or genes with a toxic product), the E. coli strain, growth conditions (aeration, temperature, culture volume, antibiotic and medium) and the culture inoculum. It also notes that some origins are temperature sensitive. Addgene adds that low- to medium-copy plasmids can still express large amounts of protein given a suitable promoter and growth conditions, so a lower copy number is not automatically a drawback.
Common misunderstandings
“The ori is the whole replication system.”
The ori is the start site. The replicon is the ori together with its control elements, and the control elements are what the copy-number regulation depends on.
“Any two plasmids can be kept in the same cell.”
Not if they share an ori. Addgene’s general rule is that plasmids with the same ori are incompatible because they compete for the same machinery, and plasmids of the same incompatibility group should not be co-transformed.
“A named copy number is fixed.”
The figures are approximate. Addgene states that actual copy number varies, and that mutations in the replicon can change it, as the pMB1 to pUC example shows.
“Every ori works in every host.”
Addgene notes the origin controls the plasmid’s host range, and that R6K-based plasmids need the pir gene. Addgene suggests asking whether the plasmid will be used only in E. coli, in Gram-negative bacteria generally, or in both Gram-negative and Gram-positive bacteria, since that affects which ori to choose.
FAQ
What is the plasmid ori?
The plasmid ori is the origin of replication: the DNA sequence where replication of the plasmid begins. It recruits the replication machinery proteins and, together with its control elements (the replicon), determines copy number and host range.
Why do plasmids with the same ori not stay together in a cell?
Addgene’s explanation is that plasmids with the same ori compete for the same replication machinery, creating an unstable and unpredictable environment. Its advice is that plasmids from the same incompatibility group should not be co-transformed, so a two-plasmid experiment needs origins that are compatible.
How many copies per cell does a plasmid origin give?
It depends on the origin and how it is regulated. In Addgene’s table the range runs from about 5 copies per cell for pSC101 to about 500-700 for pUC, and the pMB1 ori maintains about 20. Actual copy number varies.
Does the ori control which bacteria a plasmid works in?
Addgene’s Plasmids 101 introduction says the origin of replication controls the host range and the copy number of the plasmid. R6K-based plasmids, for example, require the pir gene to replicate.
Sources
- Addgene Blog, Plasmids 101: Origin of Replication, https://blog.addgene.org/plasmid-101-origin-of-replication (checked 2026-10-29).
- Addgene Blog, Plasmids 101: What is a Plasmid?, https://blog.addgene.org/plasmids-101-what-is-a-plasmid (first published 2014-01-14, updated 2020-04-14; checked 2026-10-29).
