Transformation is the genetic alteration of a cell resulting from the introduction, uptake and expression of foreign DNA. This is a common technique in molecular biology. The effect was first demonstrated in 1944 by Oswald Avery, Collin Macleod, and Maclyn McCarty, who first demonstrated gene transfer in Streptococcus pneumoniae. E. coli, which is one of the most commonly used bacteria for transformation, normally cannot take up plasmid DNA. However, exposure of the cells to divalent cations such as CaCl2 makes them more permeable to DNA. This process is relatively inefficient; electric pulses can increase the incorporation efficiency much more. An example of transformation is bT cotton, which incorporates BT genes which afford cotton plants a degree of protection against certain insect pests.

Historical context

• 1865 - Gregor Mendel develops a model of genetic heredity by the passing down of traits.
• 1928 - Frederick Griffith transforms nonpathogenic pneumococcus bacteria into a virulent variety by immersing them in heat-killed pathogenic material.
• 1944 - Oswald Avery and Collin Macleod announce that they have discovered the transforming factor - DNA.