False-colour transmission electron micrograph of plasmids (PBR 322) of bacterial DNA from the bacterium Escherichia coli (E. coli). A plasmid is a molecule of DNA that can exist apart from the chromosome in a bacterium, & which can replicate on its own. This plasmid, designated PBR 322, is the one most commonly used in genetic engineering work. Foreign DNA is spliced to it & the composite or chimeric" plasmid is then introduced into an Escherichia coli bacterium where it propagates & expresses the genetic information carried by the transplanted DNA. Magnification: X 18,450 at 35mm size."