Students should be made aware that certain viruses invade animal or plant cells without completing a destructive cycle and that these can be used as gene vectors. Generally they can carry larger portions of DNA than can plasmids. No further detail is required.

Background

A bacteriophage known as (lambda) phage has been used to modify bacteria. This is a temperate phage (one which becomes incorporated into the host's chromosome and can remain there indefinitely without having any harmful effect). The phage DNA is opened using restriction enzymes and the foreign DNA inserted. Following cell invasion, the viral DNA integrates with the host chromosome (becoming a prophage) and replicates with it at cell division.

Certain viruses can also be used as vectors to transform plant cells genetically.

Other means of getting foreign genes into cells include ballistic injection and electroporation. Both techniques are used, though not exclusively, with plant material, where the cell wall forms a barrier. Ballistic injection involves coating minute tungsten particles with the DNA to be transferred. These are then literally shot into the target cells using sophisticated methods which do not cause significant structural damage. Some of the cells take up and express the injected DNA. In clectroporation, the material is subjected to a very brief pulse of electric current which temporarily increases surface permeability so that DNA can be taken up. Used to impregnate pollen tubes, the technique has provided a route for the genetic transformation of seeds. In some cases, chemicals have provided an effective substitute for the electrical treatment.

Numerous examples of genetically transformed crop plants are available. Overwhelmingly, the introduced genes confer resistance to pests or diseases rather than impacting more directly upon yield.