There are two ways of gene transfer by which foreign DNA can be introduced into the host cells:
1. Indirect gene transfer: When the foreign DNA is inserted into host cells by the use of a vector, the method of gene transfer is called indirect gene transfer. Indirect gene transfer generally occurs through:
(i) Plant tumours: This method has been used successfully to transform plant cells, which are difficult to transform. The gene is first inserted into the Ti plasmid of soil bacterium Agrobacterium tumefaciens. This method employs the usage of tumour causing microbe Agrobacterium tumefaciens to transfer DNA, by inducing the tumour formation in plants.
(ii) Viruses: The vector is first incorporated into a virus, which is then used to infect cells, carrying the foreign gene along with its own genetic material.
2. Direct gene transfer: The method of gene transfer in which DA is inserted directly into host cell is called direct gene transfer. Direct gene transfer occurs through:
(i) Heat shock: Cells are incubated with vector in a solution containing calcium ions at 0°C. The temperature is then suddenly raised to about 40°C. This heat shock causes some of the cells to take up the vector, as sudden heat develops pores in the membrane.
(ii) Electroporation: Cells are subjected to high-voltage pulse, which temporarily disrupts the membrane and allows the vector to enter the cell.
(iii) Gene gun: This technique fires microscopic gold or tungsten particles coated with foreign DNA at cells using a compressed air gun (burst of helium).
(Iv) Microinjection: A cell is held on a glass pipette with diameter much smaller than the cell, under a microscope and foreign DNA is injected directly into the nucleus using an incredibly fine micropipette, which punctures the plasma membrane.