What is genetic modification?

DEFINITIONS

7.  The Convention on Biological Diversity[11] defines biotechnology as "any technological application that uses biological systems, living organisms or derivatives thereof to make or modify products or processes for specific use". Biotechnology has been employed for millennia: fermentation, bread-making, brewing and cheese-making were developed by the Egyptians from about 2000 BC. A definition of modern biotechnology, which is developing new techniques all the time, is difficult, but current use implies the introduction of hereditary material which could not have been achieved using traditional breeding methods[12]. The United Nations Environmental Programme guidelines for safety in biotechnology[13] define genetic modification as "modern biotechnology used to alter genetic material of living cells or organisms in order to make them capable of producing new substances or performing new functions".

DNA AND GENES

8.  The complete set of instructions for making any living organism, from the simplest bacteria to human beings, is called a genome. This contains the master blueprint for all cellular structures and activities for the lifetime of the cell or organism. It is encoded within a set of molecules called DNA (or RNA for some viruses)[14]. Each DNA molecule contains many genes which are the basic physical and functional units of heredity. Genes are "units" of DNA coding for a single product (nearly always a protein). There is a universal genetic code which applies from the simplest organisms to human beings[15]. The code allows a stretch of DNA to specify the structure of a particular protein. The amount of protein produced may vary depending on cell type, timing, environmental stress and a variety of other effects. A gene is "expressed" when the protein is synthesised[16]. Although most cells in the organism will contain the gene, the degree of expression may vary from cell to cell and tissue to tissue and thus the amount of the protein will similarly differ greatly.

9.  A crop-plant genome contains approximately 50,000 genes[17], but only about 10 per cent. of the DNA is used for coding genes. The rest includes control sequences that identify when and where particular genes are expressed in the organism and regions whose function is unknown[18]. The DNA is largely conserved during the lifetime of an organism, but is redistributed when eggs, sperm or their equivalents are formed. It is only in a clone that DNA is conserved unchanged between generations. Genes, whether introduced using genetic modification or inherently present in an organism, may be unstable, may interact with other genes and are capable of movement within the genome. In addition, their expression may be influenced by the environment.

INSERTING GENES

10.  Most organisms are only sexually compatible within their own species, and genes cannot normally be transferred from one species to another. Genetic modification allows the identification of individual genes which can then be decoded, manipulated, copied and transferred into any other organisms. In most instances the genes are transferred along with the instructions (termed "promoters") as to when in time and in what tissue they might be expressed. The techniques of genetic modification allow the transfer of genes from any organism to cells of virtually any other (using appropriate techniques) thus removing the species barrier. Genes can thus be transferred between bacteria, plants and animals (see paragraph 110). For plants and animals this is complicated by the need to introduce the genes into all the cells in the organism. Although the introduction of genes into plant cells is more difficult than it is into animal cells, the ability to regenerate many complete plants from a single cell makes plant biotechnology much easier.

11.  The proportion of the genome of the host organism that is modified is currently very small: two or three genes and their associated control elements amongst tens of thousands. The precise sequence of the genes intended to be introduced is known. The position of insertion of the inserted genes ("transgenes"[19]) is, however, not generally known. The number of copies of the insert introduced into the genome cannot currently be controlled during the insertion process. There may be other genes (or partial genes) introduced as a consequence of the technique used, but their sequence and function is known. The introduction of copies of the transgene may disrupt genes in the host genome. If the disrupted genes are essential ones, the organism may not be able to grow, and so no organism will result. If they are relatively unimportant (such as modification of colour) the unexpected modification may become apparent during the regeneration of the entire organism. Disrupted genes which are involved in processes that are only switched on during environmental stress[20] or which are expressed only under certain conditions in the lifetime of the organism may not initially be identified. The disruption may extend to changes in the timing of expression of a gene product. These changes should however surface in the process of regenerating the plant, or during the extensive breeding and selection processes used to produce a commercially useful product.

METHODS OF TRANSFERRING GENES

12.  There are two main methods for the transfer of genes into plants. The first involves the use of a soil bacterium, Agrobacterium tumifaciens, which infects certain plants. It injects a piece of DNA into the plant cell to attempt to take over the cell's protein manufacturing machinery and so produce a sugar on which the bacterium can feed. This piece of DNA is incorporated into the genome of the infected cell. Scientists use this piece of DNA by effectively hijacking it. Having removed some of the unneeded genes, they are able to insert desired genes into the vacated space. Using Agrobacterium it is possible to modify many dicotyledonous (broad leaf) plants such as potato, rape, tobacco and tomato and the technique has been adapted to work on maize, wheat and rice. The second method involves the use of "biolistics" (the "gene gun") where the desired gene package is coated around finely divided gold particles and literally fired into plant cells. A small percentage of the plant cells is transformed[21] in each case. In either method, one of the genes inserted into the plant will produce a protein that confers tolerance to a chemical that would normally kill the cell, a herbicide for example. When the chemical is administered, only those cells which have been effectively transformed and satisfactorily express the new gene product are not killed and a complete plant may be regenerated from these[22]. (This gene is termed a "marker gene" because it is used to identify the presence of the transgene.) The laboratory modification is only carried out on a suitable sub-set of varieties of the crop. These varieties are then crossed (and back-crossed) using traditional breeding technology in order to put the desired genetic material into choice varieties for agricultural production. These techniques are continuously being refined to make them more efficient and predictable.

13.  Other methods for inserting genes are now under development. One method involves using a modified plant virus to transfer the genetic material. Deleterious genes normally found in the virus are removed and genes specifying the required characteristics inserted. The subject plant, while growing, can be inoculated with the modified virus and, in a few weeks, the virus will express the desired protein in all parts of the plant.

14.  The transformation of animals is much harder, primarily because technology does not yet exist allowing the regeneration of an animal from a single cell or group of cells. The technique currently used for the modification of animals is the micro-injection[23] of DNA into embryonic cells. This provides a mosaic where only some of the cells in the resulting organism are modified and others are not, but it does provide a high yield of transformed animals (described as "chimeras" as they are only partially modified). This technique is thus not of use for the modification of agricultural animals, but could be of considerable benefit in medicine, for example in the treatment of cystic fibrosis.

WHAT FOODS CAN BE MODIFIED?

15.  All foods are of animal, plant or micro-organism origin and are therefore susceptible to modern biotechnology. Many foods are the product of traditional biotechnology, which uses micro-organisms to modify the starting material to improve taste, texture, palatability, keeping quality or safety[24]. One of the earliest modern genetic modifications enabled the production of "vegetarian cheese". The production of hard cheese used to be dependent on small quantities of rennet, scraped from the lining of dead calves' stomachs. The enzyme Chymosin is identical to rennet and is produced by genetically modified yeasts or bacteria[25]. It was introduced in commercial cheese-making in 1991 and is now used to manufacture 90 per cent. of hard cheeses (United Biscuits Q 568).

COST OF DEVELOPMENT

16.  The technology involved in genetically modifying an organism is relatively simple and inexpensive when compared to the costs of many other new technologies. Genetic modification requires a broad research and knowledge base in biology, breeding, agronomy, physiology, biochemistry and genetics. The development process from concept to commercial crop is however exceedingly expensive and takes many years.