Applied cell Biology: Genetic engineering

DNA (deoxyribonucleic acid) is the biochemical material that occurs in all living cells which carries the instructions for the maintenance and reproduction of cells. The basic structure of the DNA molecule is helical, its bases are stacked on top of each other .DNA’s four bases pair up and this system of pairing is referred to as the complementary base pairing. Genes are the chemical blueprints that determine an organism's traits. Moving genes from one organism to another transfers those traits. Through genetic engineering, organisms are given new combinations of gene and therefore new combinations of traits. This does not occur in nature and, indeed, cannot be developed by natural means.1

Figure 1 from Google images

The successful transfer of genes as mentioned above can be divided into five basic steps.

First of all the gene in particular of interest must be located within the DNA of the donor organism .This is one of the most difficult stages since a specific sequence must be selected from the total genome. Genetic material is the same in all cells of the body but differ in which genes are active; hence the products been made differ. The use of mRNA as a means of isolating the gene therefore narrows down this problem. Messenger RNA extracted can be incubated with an enzyme reverse transcriptase which catalyses the conversion of mRNA to DNA. The second stage involves cutting the required gene for recombination. DNA is a very fragile molecule which can easily be fragmented by physical means but this occurs randomly. In this case a particular gene is required; in order to cut out this particular gene from the whole DNA we need precise tools.  The enzyme restriction endonuclease acts like a pair of molecular scissors so that the required gene can be spliced out into a carrier molecule. The gene can then be cloned to get many copies of the required length of DNA.1 2

The third stage involves joining up the cut out gene into the carrier molecule (vector). In order for a foreign gene to be incorporated into a cell and to reproduce it must first be combined with a vector molecule. Two types of vectors are commonly used these are bacterial viruses or phages and plasmids. These vectors are capable of independent replication within the host cell unlike the foreign gene on its own.

In order to make joining up of the genes easier with the vector molecule, it is first treated with the same restriction enzyme. This leaves the molecules with complementary sticky ends which can be joined together using the enzyme DNA ligase. The cut molecules are mixed under conditions which can favour the annealing of the complementary strands.2

The fourth stage involves the transformation of host cells. Once the DNA has been inserted into the vector, it becomes necessary to return the DNA into a living cell. There are quite a few methods of transferring the genetically modified plasmids back into the bacteria one of which is to re-introduce the plasmid to the bacteria in the presence of calcium chloride which makes the bacterium cell wall permeable to them. Not all the cells take up the plasmids hence it is necessary to have a method of determining which ones took up the plasmid. The final stage involves selecting transformed cells, to aid this process the vectors carry ‘selection markers’ these are genes which confer identifiable chacteristics on cells that take them i.e. genes for antibiotic resistance provides an easy selection marker.  Cells with such a marker can be grown in an antibiotic medium and those that thrive on the medium must have the antibiotic resistance.3

There are many benefits that arise out of genetic engineering In terms of medical application many drugs and vaccines are now been produced with gene technology. In agricultural application scientists have developed crops that are resistant to pesticides and pests.3

One such crop of interest is the BT corn. It has been engineered to make its own pesticide. BT corn contains a gene that allows the crop to produce a natural insecticide in its pollen. This gene comes from the soil bacterium Bacillus thuringiensis, which produces the named toxin naturally. When the toxin is released in the pollen, it kills pests like the corn borer which feeds on corn reducing yield and quality hence causing economic loss to the farmers.4

However this like all other genetically modified foods has sparked fears amongst consumers. It was found by researchers at Cornell University that the pesticide produced by the BT corn although kills harmful pests that feeds on it, it kills harmless caterpillars and butterflies that improve the quality of the soil.  It is also feared that the BT crops could create populations of super-bugs that are resistant to toxins produced by these genetically engineered crops. This is because pests that survive the toxin are likely to pass on their resistance to their progeny. In time the resistance will become widespread in the pest population.4

Environmentalists are also worried that the BT crop could pollinate other plant species; this could transfer herbicide resistant to weeds making them difficult to wipe out. It is also feared that inserting combinations of plant and animal genes that have never been part of human diets could cause unpredictable health risks. Genes can cross from the food we eat to stomach bacteria; this could lead to antibiotic –resistance rendering antibiotics ineffective.7

 According to critics, the BT crop mainly benefits big biotechnology companies they develop crops resistant only to chemicals they manufacture, hence farmers have to purchase additional chemicals with the seeds. This would lead to poor farmers been forced out of business. Another area of contention concerns patents, according to the terms of patents farmers have to pay royalties to the big companies whenever a patented plant or animal produces offspring. This could lead to a few companies becoming very economically  powerful.5

Activists argue that with genetically modified foods hunger in the world can still not be eliminated. They claim that hunger is not due to scarcity but rather the distribution of the supply of food. Others also believe that genetic engineering involves tampering with nature hence scientists are trying to play God.4

The future of genetics, like any other technology, offers great promise but also great peril. Nevertheless advances in genetic engineering will undoubtedly lead to the cure of genetic diseases. Scientists however are already working on the next giant step which is to replace defective genes with healthy ones by means of gene therapy.

References

1. Bremmer M. Genetic Engineering and you. 1st Ed. London: Harper Collins Publishers, 1999.p.21-32
2. Wymer P. Genetic Engineering. Cambridge: Hobson’s publishing PLC, 1988. P.8-21.
3. Bruce D, Bruce A. Engineering Genesis. 2nd Ed. London: Earthscan publications Limited, 1999. p. 40-45.
4. The dangers of genetically modified foods. Stephen R, Sandra Smith, 2000.
5. Genesis of genetic engineering. Mathews P, 2002.
6. Advantages of genetic engineering.
7. Lapppe M, bailey B. Against the grain, 1st Ed. London: Earthscan publications Limited, 2000. p. 21-45.