The following plasmid DNA was prepared in low nuclease host bacteria and isolated through ion exchange chromatography. It is available in supercoiled form (ccc-form >95%) and, as a high-purity DNA (OD 260/280 > 1,70), it is ideal for different applications: Controlling transformation efficiency Comparison with plasmid ccc-forms of unknown size Cloning DNA-fragments Restriction (Preparing DNA-length markers) Calibrating DNA-separating or DNA-preparing processes or equipment Blocking DNA-chips with plasmid DNA of a defined sequence Plasmid DNA pBR322 lyophilized The circular sequence was numbered in such a way that 0 lies in the middle of the singular EcoR l restriction site. The copy number of pBR322 is limited by the gene product of rop to 20 copies per cell. However, by adding chloroamphenicol (final concentration 170 µg/ml) to log-phase cultures and then incubating for 8 hours, the copy number can be increased considerably. General properties: OD 260/280 : >1,70 Genomic DNA: <2 % oc form: <3 % ccc form: >95 % Base pairs: 4361