Genetic engineering. Composite image of three genetic researchers silhouetted against a coloured transmission electron micrograph (TEM) of DNA plasmids from the bacteria Escherichia coli. Plasmids are symbiotic pieces of DNA (deoxyribonucleic acid) that occur in bacteria and yeast. They are separate from the main chromosome, replicate independently, and can be transferred from one cell to another carrying genes with them. This property of plasmids is exploited in genetic engineering, and is one of the ways in which resistance to antibiotics spreads among bacteria. Magnification of plasmids: x28,000 at 6x4.5cm size. x15,000 at 35mm