Genes have been manipulated by man for a very long time, that is if selective breeding, which has been practised for centuries in agriculture and elsewhere to develop desirable characteristics in domesticated animals and plants, is to be considered as manipulation, as it rightly should. Even from the early days of Gregor Mendel, the Moravian monk and pioneer of genetic analysis, plants were bred to bring out interesting, useful and sometimes unusual traits. Many of these are now lost to classical plant breeders because of divergence of strains leading to infertile hybrids. One of the joys of genetic engineering is that in some cases, ancient genes may be rescued from seed found in archaeological digs for example, and reintroduced by transfer into modern strains. It has been proposed that the exchange of genetic information between organisms in nature is considerably more commonplace than is generally imagined (Reanney 1976) and could explain the observed rates of evolution. In bacteria, the most likely candidates for genetic transfer are plasmids and bacteriophage, and since eukaryotes lack plasmids, their most plausible vectors are eukaryotic viruses. This, of course, is in addition to DNA transfer during sexual reproduction. Current knowledge would suggest that exchange involving a vector requires compatibility between the organism donating the genetic material, the vector involved, and the recipient organism. For example, two bacteria must be able to mate for plasmid transfer to take place, or if a virus is involved as a vector, it must be able to infect both the donor and recipient cells or organisms. However, there is evidence to suggest that this view is somewhat na¨ıve and that there is considerably more opportunity for genetic exchange between all cells, prokaryotic and eukaryotic, than is popularly recognised.
Bacteria are notorious for their ability to transfer genes between each other as the need arises thanks to the location on plasmids of most of the gene groups, or operons, involved in the breakdown of organic molecules. Strong evidence for the enormous extent of these ‘genomic pools’ comes from analysis of marine sediment (Cook et al. 2001). Throughout this book, the point has been made that micro-organisms involved in remediation do so in their ‘natural’ state largely because they are indigenous at the site of the contamination and have developed suitable capabilities without any external interference. However, sometimes after a sudden contamination such as a spill, microbes are not able to amass useful mutations to their DNA quickly enough to evolve suitable pathways to improve their fitness for that changed environment, and so they may be ‘trained’ by the artificially accelerated expansion of pre-existing pathways. The final option is that they may be genetically engineered. Organisms which represent the ‘norm’, frequently being the most abundant members occurring in nature, are described as ‘wild type’. Those with DNA which differs from this, are described as mutant. Alteration can be by the normal processes of evolution which constantly pro-duces mutants, a process which may be accelerated artificially, or by deliberate reconstruction of the genome, often by the introduction of a gene novel to that organism. This latter route is the basis of genetic engineering (GE) which has several advantages over traditional breeding or selection techniques. The process is specific, in that one gene, or a selected group of genes, is transferred and so the mutation is quite precise. There is flexibility in the system in that, depending on the modifications made to the genome, a new product may be produced or the level of expression of the existing product or products may be altered in quantity or proportions to each other. Another advantage often quoted is that GE allows genes to be transferred between totally unrelated organisms. The preceding dis-cussion suggests that this is not a phenomenon unique to GE, but it is at least defined and specific.
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