DNA plasmid and enzymes. Coloured 3-D atomic force micrograph of an opened pRH3 plasmid (mauve) of DNA (deoxyribonucleic acid) bound to EcoKI restriction-modification enzymes (bright peaks). A plasmid is a loop of DNA that can replicate and express genes in a cell independently of the cell's chromosomal DNA. They can be used in genetic engineering to introduce new genes to cells. EcoKI is an enzyme that recognises and breaks up foreign DNA in E. coli bacteria, as a defence against pathogenic viruses. Atomic force microscopes make an image by moving a sensitive probe over a surface. Magnification: x63,000 at 6x7cm size.