DNA plasmids. Computer artwork of DNA plasmids from bacteria. Plasmids are symbiotic pieces of DNA (deoxyribonucleic acid) that occur in bacteria and yeasts. They are separate from the main bacterial chromosome, replicate independently, and can be transferred from one cell to another during a process known as conjugation. Such transfer of plasmids between bacteria can alter the genes expressed by the recipient cell. This property of plasmids is exploited in genetic engineering and is one of the ways in which resistance to antibiotics spreads among bacteria.