Why pBR322 plasmid have high copy number?
I think pBR322 has a replication module from E coli plasmid
colE1 ,which permits plasmid replication even when chromosome
replication and cell division are inhibited by amino acid
starvation and chloramphenicol, as a result, under such condition
each cell accumulates several thousands copies of the plasmids up
to 3000, so that one litre of bacterial culture easily yields a
milligram of plasmid DNA.