in biology, one of several processes by which genetic material in the form of “naked” deoxyribonucleic acid ( DNA) is transferred between microbial cells. Its discovery and elucidation constitutes one of the significant cornerstones of molecular genetics. The term also refers to the change in an animal cell invaded by a tumour-inducing virus.

The study of transformation dates to the late 1920s, when an English physician, F. Griffith, discovered that pneumococcal cells (Streptococcus pneumoniae (pneumococcus)) could convert from a harmless form to a disease-causing type. He noticed that pneumococci may or may not have a capsular covering. Those cells with a capsule (forming smooth colonies) caused disease in mice; those lacking a capsule (and forming rough-surfaced colonies) were harmless. A mixture of living, nonencapsulated cells and heat-killed, capsulated cells, when inoculated into mice, caused disease. Living, encapsulated cells (pathogenic) were created by a “transforming principle” liberated from the dead cells to the living cells. The transformation was heritable. In 1943 a group of investigators at the Rockefeller Institute, New York City, identified that “transforming principle” as DNA.