Genetic Engineering is the direct alteration of an organism's genetic code. It is a technology that has great potential, but can be used both ethically and unethically. Genetic engineering usually consists of two stages: isolation of the target gene(s), and insertion into an organism, using a vector.
Isolating the gene
The first stage of giving an organism a certain genetic property, is to work out which gene
(s) code for the property, and to obtain the DNA
for that gene
. A number of different ways of doing this exist, depending on how much is known about the gene
Inserting the gene
- Direct identification
- By analysing the genetic differences between organisms with or without the desired properties, the location of the gene can be determined, and it can be cloned and used.
- Reverse transcriptase
- If the required gene is known to be for a specific protein, the mRNA for that protein can be used to create the DNA for that gene by making a DNA copy with reverse transcriptase.
- If the amino acid code of the protein required is known, the code can be translated back into DNA, and a DNA gene for the code can be artificially synthesised.
This depends on the type of organism that is being modified:
- Bacteria are by far the easiest organisms to genetically engineer: they contain a ready made vector: the plasmid, which is easy to modify and use, they are easily grown in laboratory conditions, the effects of any modification can be detected much more easily than with a large organism and the cell processes, and in some case the genome, are known in detail. Two main ways of modifying bacteria exist:
- DNA can be easily inserted into bacteria by using plasmids, which are short pieces of circular DNA. Adding plasmids to the medium of a colony of bacteria and giving them a small electric shock will allow some of the plasmids to enter the bacteria and be expressed. An antibiotic resistance gene is usually contained with the vector so that by adding the antibiotic to a batch of bacteria that has had plasmids added, the ones in which the plasmid is expressed will survive. Plasmids are the most reliable gene vector for any kind of genetic engineering, and the most commonly used.
- Larger lengths of DNA can be inserted using phages, a type of DNA virus, although the efficiency of plasmids make phages an unusual choice. Some of the DNA form the phage is replaced with the DNA from the target DNA, so that as it inserts its DNA, it also inserts the target DNA. This is less efficient and more complicated than plasmids, as the virus itself has to be manufactured.
The genetic modification of bacteria has led to a number of successes, most notably the successful production of artificial human insulin by the e.coli bacteria. Once the insulin protein was isolated, it was inserted into the e.coli, along with a lac promoter, which is switched on when the bacteria is in a medium containing lactose, allowing the expression of the gene to be controlled. This has directly affected the lives of over a million people, and is probably genetic engineering's biggest success.
The genetic engineering of plant
s is harder than that of bacteria
because plants are much more complex organisms, and therefore the effect of any genetic engineering is harder to predict, plants have cell wall
s, which make it more difficult for simple vector
s to enter, and plants do not have plasmid
s. The potential benefits of genetically engineering plants are huge. Three main methods are used to insert DNA into plants:
- The gene gun
- The gene gun is a simple but effective method. It involves coating several hundred small gold pellets with the target DNA and firing the pellets down a tube at a target containing the plant material to be modified. As the pellets hit the sample and pass through the cell walls, the DNA comes off, and some of it can enter the nucleus of the cells, where it can be expressed.
- The other major plant vector is the soil bacterium Agrobacterium tumefaciens, which normally infects plants, causing crown gall disease. When it infects a plant, it normally produces T-DNA, which normally enters the surrounding plant cells and modifies them to divide uncontrollably and produce an environment inside the plant where the bacteria can live. In genetically modified Agrobacterium, however, the plasmid that contains the T-DNA, has had the DNA that causes the plant cells to divide uncontrollably replaced with the target DNA, which is then inserted into the plant cells.
- In a similar manner to the phages in bacteria, viruses that insert target DNA into plant cells are being developed.
Several genetically modified plant strains are now being developed. One of the most important is "golden rice". Normal rice is a major staple food source, but is short in vitamin A, which can lead to blindness and other health problems. Golden rice is a GM rice variety containing a gene for beta-carotene, the pigment that makes carrots orange, which contains vitamin A. Golden rice is the genetics industry's PR project, and will be distributed free to Third World farmers. Although it could be a great success, it could also be useless, as many people believe the vitamin A problem could be countered just by better dietary information.
s and other vertebrate
s are the hardest organisms to genetically modify, both because they are very complex, and because they have advanced immune system
s that will kill genetically different cells. Also, unlike bacteria and plants, they cannot be grown from a single modified cell. Mammalian genetic engineering focuses more on curing existing genetic defect
s than improving the characteristics of an organism. Two vectors exist for engineering mammals:
- The most likely candidates for a suitable vector are viruses, as they are already capable of inserting their DNA/RNA into the human genome, but finding a virus that is not dangerous and is effective has been difficult. Candidates at the moment include retroviruses, adenoviruses and adeno-associated viruses.
- Nonviral vectors
- Nonviral vectors are artificial complexes that function in a similar way to viruses, transporting DNA into the cells. At the moment they are usually cationic lipid complexes. They are less likely to be detected by the immune system than viruses, but are much less efficient at the moment.
The most successful genetic engineering on a mammals has been the gene therapy for X-SCID on humans. X-SCID is X-linked Severe Combined ImmunoDeficiency, the condition that produces "babies in a bubble". Bone marrow cells are taken from the sufferer and modified with a retrovirus containing the target gene. The bone marrow cells which have been successfully altered are then transplanted back into the sufferer. Several clinical trials were conducted, most of them successful, but one of the subjects has now developed leukaemia, a cancer of the bone marrow, which could have been a result of the gene therapy. The future of gene therapy trials are now in doubt.
The examples listed above are the tip of the iceberg. Many more GM organisms are being created, and the debate between pro and anti GM groups is becoming increasingly important as more and more discoveries are made and technologies developed. Many people believe that GM organisms are harmful to the environment, and little of the genetic engineering technology is being used to help those it could really help, such as those in the Third World, instead, companies are focusing on ideas that will make them a large profit. The depth of this debate is discussed on other nodes, such as genetically modified food and I advise you to look at them, but ethics is inextricably linked with this debate.
- Transgenesis:Applications of gene transfer, J.A.H Murray
- Advanced Biology, Jones and Jones