Genetic engineering may be defined as a technique of synthesizing an artificial gene, repairing of a gene, combining DNA from two organisms and manipulating the artificial genes together with recombinant DNA for improvements of microbes, plants, animals and human beings. The technology involves combining DNA from two different organisms to generate a recombinant DNA. This involves manipulation or engineering of two DNA involved and therefore the term “genetic engineering” has come into existence.
There are two main discoveries to the birth of this field of research. One is the discovery of restriction enzymes elaborated mostly by certain bacteria. Restriction enzymes are endo-nucleases which cut DNA at specific sites into fragments containing identifiable genes. The second break through is because of the development of methodologies to produce large quantities of these DNA fragments containing genes using vectors or cloning vehicles.
Techniques of Genetic Engineering
1. At the first step, DNA molecules are cut into specific fragments containing identifiable genes by using restriction enzyme endonuclease.
2. Vectors are used for transferring the DNA fragment to a suitable recipient bacterium. Vectors may be plasmids, bacteriophages, cosmids and others.
Plasmids are small, circular DNA molecules that occur naturally in bacteria. They carry genes for sexuality, antibiotic resistance etc. but not any vital gene, so that a cell can survive even without them. Plasmid DNA replicates independently of the main genome and being small, can easily come out of or get into a cell.
3. The restriction endonuclease used to cut the foreign DNA is also used to splice the plasmid at specific point in such a manner so as to have sticky ends in both the molecules.
4. The free sticky ends of the plasmid DNA and the foreign DNA serve as convenient points for their complementary pairing.
5. The gaps are then sealed by an enzyme ligase, thus making a circular DNA piece which contain the plasmid genes as well as a piece of foreign DNA. The product is referred to as recombinant DNA.
6. Such a recombinant DNA can be introduced into a bacterial cell as plasmid, where it can replicate and express itself. When such new foreign DNA fragments are introduced into the host cells, such cells are said to be transformed and the process is known as transformation.
7. The foreign DNA can then be released from the recombinant plasmid of clone cells once again by the use of restriction enzymes. Thus large quantities of foreign genes can be isolated by this technology.
Using this technique, the ribosomal genes of Xenopus (a toad) can be transferred to the cells of bacteria E.coli. It has also been possible to transfer the globin gene (the gene which codes for the protein part of the haemoglobin) of a rabbit and the insulin gene of a rat to the same bacterium. The insulin was obtained by cloning in 1982 and is sold under the name humulin. The world’s first genetically engineered vaccine against the dreaded hepatitis by virus (HBV), causing 80 percent of liver cancer has been prepared by this technology.
All the chromosomes of the cell nucleus are collectively known as the genome of that organism. The genome containing many genes can thus be cut into smaller DNA fragments containing one or a few genes and such fragments can be cloned into bacterial, yeast, insect, plant or animal cells. The bacterium generally used in genetic engineering is E.coil. Each cell thus acquires a particular DNA fragment containing one or few genes. When this cell multiplies a group of cells containing the same foreign DNA fragment is formed. This is referred to as a clone of cells. The process is called cloning the DNA. Several clones of cells, each containing one or a few foreign genes are finally obtained. They represent almost all the genes of an organism and this is referred to as the gene library of that organism. From this gene library, it is possible to identify a clone containing the desired gene. This gene can also be isolated in large quantity by growing the particular clone in large culture.
Significance of Genetic Engineering
1. Genetic engineering puts us at the threshold of a new form of medicine, gene therapy, to crippling hereditary diseases like hameophilia, phenylketonuria etc.
2. By this technique, vitamins, hormones, antibodies and vaccines can be produced.
3. A number of valuable genes such as Nifgene (Nitrogen fixation gene) can be introduced into other bacteria to improve the soil fertility and crop production.
4. Efforts have been made to replace defective genes with normal or healthy genes.
5. It also helps in understanding the concept of exons and introns present in the eukaryotic genes.
6. It may well be possible to produce plants and animals of totally a new design and to tailor their characteristics according to will.
7. It also permits a thorough study of the nature and function of the hereditary material and provides a way to find the location of specific genes within the chromosome.
Possible disadvantages of Genetic Engineering
1. There is a positive danger that the manipulation of genes might by accident, result in the origin of new kinds of diseases.
2. It may result in the origin of new kinds of organisms, containing fatal genetic elements, which may escape and contaminate the entire earth and may not reveal its presence until its deadly work is done.
3. Many drugs such as antibodies may become ineffective if the bacteria acquire resistance due to uncontrolled recombinant DNA.
4. There is also the fear of some possibility that the politicians may misuse this technique to serve their own ends to create Hitler type ruthless dictators or Polpot type political murderers.