Background
The origin of replication determines the vector copy number, which could typically be in the range of 25–50 copies/cell if the expression vector is derived from the low-copy-number plasmid pBR322, or between 150 and 200 copies/cell if derived from the high-copy-number plasmid pUC. The copy number influences the plasmid stability, i.e. the maintenance of the plasmid within the cells during cell division. A positive effect of a high copy number is the greater stability of the plasmid when the random partitioning occurs at cell division. On the other hand, a high number of plasmids generally decreases the growth rate, thus possibly allowing for cells with few plasmids to dominate the culture, since they grow faster. There appears to be no significant advantage of using higher-copy-number plasmids over pBR322 -based vectors in terms of production yields.
The origin of replication also determines the plasmid's compatibility: its ability to replicate in conjunction with another plasmid within the same bacterial cell. Plasmids that utilize the same replication system cannot co-exist in the same bacterial cell. They are said to  belong to the same compatibility group. The introduction of a new origin, in the form of a second plasmid from the same compatibility group, mimics the result of replication of the resident plasmid. Thus any further replication is prevented until after the two plasmids have been segregated to different cells to create the correct prereplication copy number.
 

1. Replication and control of circular bacterial plasmids. del Solar G, Giraldo R, Ruiz-Echevarria MJ, Espinosa M, Diaz-Orejas R. Microbiol Mol Biol Rev 1998 Jun;62(2):434-64
2. Vectors (ppt)

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