Plasmids

A plasmid is a circular extrachromosomal circular DNA containing from 1000 to 200,000 bp, and reproducing independent from the chromosomal DNA. Plasmids are inherited by daughter cells after cell division. Plasmid replication can be inhibited by curing the cells with compounds such as ethidium bromide. Some of the plasmids may exist in a limited number (1–3) of copies (stringent plasmids) or relatively large number (10–220) of copies (relaxed plasmids).