Genetic engineering involves deliberate DNA manipulation in organisms to alter their genes. Although the organisms whose genes are being altered may not be microbes, but the substances and techniques involved are obtained from microbes and adapted for use in more complex organisms.

Historical Background

The term genetic engineering initially was used for various techniques used for modifying or manipulating organisms through heredity and reproduction processes. Genetic engineering involves artificial selection and also all the interventions of biomedical techniques (artificial insemination), in vitro fertilisation (e.g., test-tube babies); cloning, and gene manipulation.

In the 20th  century, the term genetic engineering was used to indicate more specific methods of recombinant DNA technology (or gene cloning), in which DNA molecules obtained from two or more sources are combined in vivo (within cells) or in vitro (outside cells) and then inserted into the host organisms for propagation.

The techniques of recombinant DNA technology were developed with the discovery of restriction enzymes by Werner Arber (a Swiss microbiologist) in 1968. The next year Hamilton O. Smith (an American microbiologist) purified type II restriction enzymes having the ability to cleave a specific site within the DNA (in contrast to type I restriction enzymes that cleave DNA at random sites), and thus essential to genetic engineering.

Daniel Nathans (an American molecular biologist) helped in modifying the DNA recombination technique in 1970-71 and demonstrated that type II enzymes could be useful in genetic studies. Genetic engineering based on recombination was pioneered in 1973 by Stanley N. Cohen and Herbert W. Boyer (American biochemists), who were the first to cut DNA into fragments, rejoin different fragments, and insert the new genes into E. coli bacteria, which then reproduced.

Basic Principles

Gene cloning involves inserting a specific piece of ‘desired DNA’ into a host cell in such a manner that the inserted DNA is replicated and handed onto the daughter cells during cell division. The factors involved in gene cloning are:

1. Isolation of the gene to be cloned.
2. Insertion of the gene into a vector (piece of DNA) which allow it to be taken by bacteria and replicate within them as the cells grow and divide.
3. Transfer of the recombinant vector into bacterial cells by transformation or infection with viruses.
4. Selection of the cells containing the desired recombinant vectors.
5. Growth of the bacteria, that can be continued indefinitely, to give the required cloned DNA.
6. Expression of the gene to get the desired product.

Process

The recombinant DNA technology mainly involves insertion of foreign genes into the plasmids (small rings of DNA) of common laboratory strains of bacteria. Plasmids are not the part of bacterium’s chromosome (the main source of genetic information in the organisms).

However, they can direct protein synthesis, and are reproduced and passed on to the bacterium’s progeny (like chromosomal DNA). Thus, foreign DNA (e.g., a mammalian gene) can be inserted into a bacterium to obtain immeasurable number of copies of the inserted gene. Also if the inserted gene has the ability to direct protein synthesis, the modified bacterium will produce the protein specified by the foreign DNA.

A subsequent generation of genetic engineering techniques that emerged in the early 21st  century centred on gene editing. Gene editing is based on CRISPRCas9 technology, and the researchers by using it can customise a living organism’s genetic sequence by making specific alterations in its DNA. Gene editing can be used for the genetic modification of crop plants and livestock and of laboratory model organisms (e.g., mice). It is also used in gene therapy for humans as it can correct the genetic errors associated with disease in animals.

Applications

Genetic engineering has improvised the understanding of many theoretical and practical aspects of gene function and organisation. Through recombinant DNA techniques, researchers can produce bacteria which can synthesise human insulin, human growth hormone, α-interferon, hepatitis B vaccine, and other medically useful substances.

Plants can be genetically altered to enable them to fix nitrogen. Genetic diseases can be corrected by replacing the dysfunctional genes with normally functioning genes.

However, special attention has been paid on such achievements as they might introduce unfavourable and dangerous traits into microorganisms that were earlier free of them, e.g., resistance to antibiotics, production of toxins, or a tendency its use to alter traits, like intelligence and beauty.

Genetically Modified Organisms (GMOs or transgenic organisms) contain genes from different organisms, and are the sources of:

DNA: GMOs can be made such that a DNA piece can be easily replicated, thus providing a large source of that DNA. For example, a gene associated with breast cancer can be cut into the genome of E. coli, allowing the rapid production of the gene so that it may be sequenced, studied, and manipulated, without requiring repeated tissue donations from human volunteers.

Protein: Since microbes replicate rapidly, it can be advantageous to use them for manufacturing the desired proteins. Given the right promoters, bacteria will express genes for proteins not naturally found in bacteria, such as cytokine. Genetically engineered cells are used to make various proteins essential for humans, e.g., insulin or human growth hormone.