Transformation - Transformation was first discovered by Griffith in 1928 in Diplococcus pneumoniae. But Avery, Macleod and McCarthy explored the exact phenomenon and principle in 1944. In transformation, naked DNA molecules are taken in by bacterial cells. These DNA molecules replace homologous segments of the chromosomes of recipient cells.

The uptake of DNA molecule by recipient bacteria is an active, energy requiring process. It does not involve passive entry of DNA molecules through permeable cell walls and membranes. Thus, transformation does not occur naturally in all species of bacteria.