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[7] Genetic modification", or more specifically, "transgenic technology" is a set of techniques for manipulating, or engineering the genetic material, DNA, enabling molecular geneticists to cut and join, mutate, copy and multiply genes, and most importantly, transfer genes between species. A major vehicle for genetic manipulation is artificially constructed, parasitic genetic elements, or vectors, used to multiply copies of genes, and to carry and smuggle genes into cells, which would normally exclude them. Once inside the cells, these vectors slot themselves into the host's genome, to genetically transform the cells to make transgenic organisms. The integration of the vector DNA, as indeed any foreign DNA into the host-cell genome is a very imprecise, random process, known to cause many genetic disturbances with harmful or lethal effects. Vectors used in transgenic technology are usually a mosaic recombination of natural genetic parasites from different sources, including viruses causing cancers and other diseases in animals and plants, with their pathogenic functions "crippled". The vector most widely used in plant genetic manipulation is derived from a tumour-inducing plasmid carried by the bacterium Agrobacterium tumefacient ENS. In animals, vectors are constructed from retroviruses causing cancers and other diseases. Thus, a vector currently used in fish has a framework from the Moloney Murine leukemic virus, which causes leukaemia in mice but can infect all mammalian cells. It also has bits from the Rous sarcoma virus, causing sarcomas in chickens and from the vesicular stomatitis virus causing oral lesions in cattle, horses, pigs and humans.

[8] There is serious concern about the dangers of using genetically engineered viruses as delivery vehicles (vectors) in the generation of transgenic plants and animals. This could destabilize the genome and lead to horizontal gene transfer to other species, including mammals. This may cause dangerous new diseases, resistance to antibiotics, and severe immune reactions. Genetic engineering also interferes with RNA editing and molecular folding, which may cause the information of prion-based diseases similar to BSE - mad cow disease.

How to get the gene into the other cell.

[9] There are different ways to get agents from A to B or to transform a plant with a "new" gene. A vector is something that can carry the gene into the host, or rather into the nucleus of a host cell. Vectors are commonly bacterial plasmids. One of the methods used is the "shotgun technique" also known as "bio-ballistics", which blindly shoots masses of tiny gold particles coated with the gene into a plate of plant cells, hoping to land a hit somewhere in the cell's DNA.

What is a plasmid?

Plasmids can be found in many bacteria and are small rings of DNA with a limited number of genes. Plasmids are not essential for the survival of bacteria but can make life a lot easier for them. Whilst all bacteria -no matter which species - will have their bacterial chromosome with all the crucial hereditary information of how to survive and multiply, they invented a tool to exchange information rapidly. If one likens the chromosome to a bookshelf with manuals and handbooks, and a single gene to a recipe or a specific building instruction, a plasmid could be seen as a pamphlet. Plasmids self-replicate and are thus easily reproduced and passed around. Plasmids often contain genes for antibiotic resistance. This type of information, which can easily be passed on, can be crucial to bacterial strains, which are under attack by drugs, and is indeed a major reason for the quick spread of antibiotic resistance.

Working with plasmids

Plasmids are relatively small, replicate very quickly and are thus easy to study and to manipulate. It is easy to determine the sequence of its DNA, that is, to find out the sequence of the letters (A, C, G and T) and number them. Certain letter combinations such as CAATTG - are easy to cut with the help of specific enzymes. (Proteins) These cutting enzymes, called restriction enzymes, are part of the Genetic Engineering 'tool-kit" of biochemists. So if I want to splice a gene from a fish into a plasmid, I have to take the following steps: I place a large number of a known plasmid in a little test tube and add a specific enzyme that will cut the plasmid at only one site. After an hour I stop the digest, purify the cut plasmid DNA and mix it with copies of the fish gene; after some time the fish gene places itself into the cut ring of the plasmid. I quickly add some "glue" from my 'tool-kit" - an enzyme called ligase - and place the mended plasmids back into bacteria, leaving them to grow and multiply. But how do I know which bacteria will have my precious plasmid? For this reason I need marker genes, such as antibiotic resistance genes. So I make sure my plasmid has a marker gene before I splice my fish gene into it. If the plasmid is marked with a gene for antibiotic resistance I can now add the specific antibiotic to the food supply of the bacteria. All those, which do not have the plasmid, will die, and all those that do have the plasmid will multiply.

Reference:

[7] PERILS AMID PROMISES OF GENETICALLY MODIFIED FOODS - Dr Mae-Wan Ho Biology Department, Open University, U.K. November 1996

[8] Ref: (Green, A. E. (1984) Science 263:1423 Osbourne, J.K. et al (1990) Virology 179:921;Mae-Wan Ho (1996) Biology Dept, Open University

[9] What is Genetic Engineering? By Ricarda Steinbrecher, The Women's Environmental Network Synthesis/Regeneration 18 (Winter 1999)

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