Genetics is the science of heredity. Long before Watson and Crick described the helical structure of DNA, man has been changing the genetic make-up of plants and animals to his advantage. By selectively breeding them over thousands of years, we have altered their characteristics to suit our needs. The food crops eaten to-day have been changed to grow larger, taste better and survive different growing conditions than their wild originals, eg. all of the cultivars of cabbages derive from a wild species which still grows around our sea shores, but it is smaller and more bitter.
Much of this domestication was done before Gregor Mendel explained the mechanisms of heredity. However, improving the flavour and appearance has probably removed any natural defenses the wild form might have had and growing crops as a monoculture makes them more vulnerable. So growers have had to replace these with pesticides and these cause other problems. Genetically Modified Organisms (GMOs) are the latest idea to overcome some of these difficulties by altering the genetic makeup of cells to protect from the pest directly or allow the use of pesticides in a more targeted way.
The genetic material is found in the nucleus of cells in the form of chromosomes. These are usually arranged in pairs and each chromosome is made up of a chain of genes which dictate the processes within the cell, and ultimately the whole functioning of the organism that they make up. Cells divide in order that the organism can grow or to replace those that are no longer functioning fully. During this cell division the chromosomes unwind and are copied to provide the nuclear material for the new cells. Also during reproduction special cells called gametes are produced where the chromosomes do not pair up and they have half the number carried by normal cells. So they have to obtain the other half of the full complement from somewhere else and in the higher plants and animals this is usually from another individual. This means that small variations in the genetic material which occur from time-to-time in members of a species can be passed on to the next generation. This variation can either come to the fore or may die out depending on whether it brings an advantage or not.
These are the variations that breeders look out for, or in the wild, environmental conditions can select them depending on the changes they bring to a species - this is the mechanism first explained by Charles Darwin in his book The Origin of Species by Means of Natural Selection.
This Genetic Modification (GM) can also involve genes which are not from the same species. The basic building blocks of genes are four nitrogen-based compounds called nucleotides - adenine (A), guanine (G), cytosine (C) and thymine (T), which are assembled in a unique combination in every gene. These nucleotides are the same in every plant and animal, so they are interchangeable.
If a plant shows resistance to a disease it is probably due to something in its genetic make-up and this resistance can be transferred to another plant by inserting the gene responsible into the second one. This is done by chemically "clipping" the required gene from the chromosome of the donor plant and, using a vector, it is carried into the nucleus of the target cells. The newly modified cells are then grown on and they differentiate as they normally would into a mature plant in the same way cloned plants are, using a micro-culture technique.
In the modification of dicotelydenous plants the vector is usually a bacteria called Agrobacterium tumefaciens, a natural parasite which invades plant cells and has special structures called plasmids which can insert some of its genetic material into the host. By placing the required gene into the plasmid, it is added to the genetic material of the plant to be modified. A more advanced technique of gene transfer is by using a gene gun where the "clipped" genes are wrapped around gold particles and the cell nucleus is bombarded with these.
Gene editing is another method of altering the characteristics of an organism. After determining the the gene that is causing a problem, or that could be enhanced to give resistance to a disease or grow larger crops, the DNA can be edited using a system found in bacteria called CRISPR/Cas9 or more usually just CRISPR. (The term is short for Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR associated protein 9). The bacteria use the Cas9 enzyme to cut up the DNA of invading viruses to kill them the CRISPR part is a sequence of nucleotides that guide the enzyme to the part of the DNA that is to be cut. The same mechanism can be used to change the nucleotides of the target genes of an organism. After identifying the part of the gene to be altered a guide can be constructed to match the sequence and when it links to the correct portion the Cas9 enzyme snips the DNA. The cell then tries to mend the cut using DNA in the vicinity and if a new portion that would improve the characteristics is introduced at this point it may be included. If there is no DNA around the cell will join the cut ends with the targeted gene missing therefore removing a faulty bit. It would not be practical to alter the genes of a developed organism so the technique is used at the single cell embryo stage or in a stem cell.
 |
|
Eighty percent of cotton and soya beans grown in USA are GM crops, where they have been grown openly for about ten years. Also half of the arable land in Argentina is used in their production - mostly soya beans which are exported world-wide for animal feeds, even to countries opposed to growing them themselves.
Cotton is usually attacked by the Pink Bollworm (Pectinophora gossypiella), a caterpillar which infests the developing seedhead which produces the pappus that is spun into yarn. The growth is stunted and ruined for commercial use. The new crop has been modified to produce Bt-toxins. These natural insecticides are produced by Bacillus thuringiensis subsp. kurstaki and the genes for this property are extracted from them for inclusion in the modified plants. Similar protection has been given to GM Maize.
Another bacterial gene protects plants from Glyphosate the popular herbicide, by blocking the damage it does to the EPSP synthase enzyme, so weed-free crops can be grown. Roundup Ready Oil-seed Rape has this gene present.
There are advantages that can be seen for producing Genetically Modified Organisms (GMOs). The use of harmful pesticides could be greatly reduced or eliminated in some cases and these are usually manufactured from fossil fuels. Artificial fertilizers are also made from oil, so reduced requirements would be desirable. Crops could be more easily managed, further reducing the fuel demand. The benefits from plants which are seasonal can be transferred to ones which can be grown all year round, eg, genes from fruit rich in antioxidants placed in tomatoes.
There are other uses of the technology in medicine to produce drugs. Modified bacteria are used to make proteins from human genes so insulin, growth hormones and clotting factors can be manufactured in a purer form with fewer side effects.
Since a modified plant can go on to produce the gametes for reproduction, the extra genes will be included and carried to any offspring. This is where problems could arise as pollen floats freely and may end up in nearby plants closely related to the GM ones. If the resistance to a herbicide transfers to wild plants by this 'gene flow', new superweeds could result. The Bt-toxins mentioned above have also been shown to affect soil chemistry and if the ability to produce them is transferred to the wild, non-pests would be affected. Also there are strains of the target pests which are developing resistance to the Bt-toxins so the protection is reduced.
There are other concerns that the gene gun technique causes some changes and mutations to the DNA of the target nucleus. The modified cells can go on to manufacture new proteins with unknown properties. There are fears that these may be toxic or give rise to new diseases and allergies.
The GM crop seed is produced by large multinational companies such as Monsanto, which have spent many millions developing them so every aspect of the process is protected by patents. This gives them total control from planting to harvest including the supply of any chemical treatments, and it is illegal to collect seed for re-sowing. It is claimed that such crops are the answer to starving nations as characteristics like drought resistance and growing with low soil fertility can be overcome.
There is also a tendency to solve problems that do not exist. Just because you can create tomatoes that produce the antioxidants from blackcurrants doesn't mean that they are needed - why not eat the blackcurrants instead. Also there are other breeds of animals and varieties of plants which have resistance to diseases and are adapted to thrive in harsher conditions, that can be cross-bred to improve existing stock. This is a good reason to preserve the older breeds and varieties for future use.
The arguments for and against continue, but there are still many unknowns. Biological controls were first introduced many years ago as the answer to pests and diseases, but many of the earlier examples went on to create more problems than they solved. Organic growers are particularly concerned as their crops are contaminated by the stray pollen.
