Transformation: A Means of Genetic Transfer

Transformation is the genetic alteration of the cell which is the result of uptake, incorporation and expression of exogenous genetic material that is taken up through cell membrane. It occurs in bacteria and some species in a natural manner. Genetic material can be exchanged between two bacterial cells either by conjugation or transduction. Conjugation involves direct transfer of genetic material from one cell to the other through intimate contact. In the process of transduction bacteriophages are known to transfer the foreign genetic material into the host cell. Introduction of the foreign DNA into the eukaryotic cells is known as transfection. Transduction can also be used as a tool for the introduction of foreign DNA into a non-bacterial cell which may be either a plant cell or an animal cell.

Transformation did not become the regular procedure until 1972 when Stanley Cohen, Annie Chang and Leslie Hsu successfully transformed Escherichia coli by treating the bacteria with calcium chloride. Transformation by using electroporation was developed in the late 1980s when the efficiency and the number of bacterial cells can be increased through transformation. Transformation of plant cell and animal cell was also started and the first transgenic mouse was produced in 1982. In 1907 a bacterium named as Agrobacterium tumefaciens was found which was responsible for causing tumors in the plants and the tumor inducing agent was found to be DNA plasmid known as Ti-plasmid. Bacterial transformation may be called as a stable genetic change brought about by uptake of foreign DNA and competence may be defined as the state of being able to take the exogenous DNA from the environment. Competence may be natural or artificial. About 1% of the bacterial population is able to take the DNA naturally under the laboratory conditions and many more species are known to take up the foreign DNA in their natural environment. These bacteria carry a set of genes that provide the protein machinery to bring the DNA across the cell membrane. The transfer of genetic material between two different strains of bacteria is called as horizontal gene transfer.

Artificial competence can be carried out under laboratory conditions and the cell is allowed to take the genetic material passively under the controlled environment. Calcium chloride transformation is the common method used for inducing artificial competence. Chilling the cells in the presence of divalent ions like the calcium cells make them ready for the intake of plasmid DNA. The cells are then incubated on ice along with the DNA and then heat shocked. This step allows the DNA to enter into the cells. This method is very effective for the circular DNA. An excellent preparation of competent cells results in the generation of approximately 108 colonies per microgram of plasmid used. This process of genetic transfer fails to work for the linear DNA like the chromosomal DNA as the cell's native exonuclease enzymes degrade the linear DNA. Cells that are naturally competent are usually transformed more effectively with the linear DNA than with the plasmid DNA.

Electroporation is another method of inducing artificial competence. In this process cells are briefly shocked with electric field of 10-20 kV/cm which forms holes in the cell membranes and the plasmid DNA can enter in these cells easily. After electric shock the holes are soon closed by the repair mechanism of the cell membrane. The efficiency with which a competent culture can take up the foreign genetic material and express its genes is known as transformation efficiency. The plasmid DNA molecule that has been introduced in cell must contain an origin of replication so that the DNA can replicate on its own without utilization of the machinery of the host's chromosome. Transformation usually produces a mixture of few transformed cells and a large proportion of non-transformed cells so it is very important to identify the cells that have acquired the plasmid. Only the transformed cells are able to grow in the medium containing antibiotic and the non-transformed cells get killed.