Genetic engineering is a branch of genetics concerned, in its broadest sense, with the alteration of the inherited, genetic material carried by a living organism, in order to produce some desired change in the characteristics of the organism.
In practice, the main application of genetic engineering to date has been to mass-produce a variety of substances – all proteins of various sorts – that have uses in medical treatment and diagnosis. The function of any gene is to control the production of a particular protein in a living cell. If the gene responsible for synthesizing a useful protein can be identified, and if such a gene can be inserted into another cell that can be made to reproduce rapidly, then a colony of cells containing the gene can be grown. The colony will then produce the protein in large amounts.
Why genetic engineering is done
Genetic engineering has been used for producing some human hormones (notably insulin and growth hormone), some proteins for use in vaccines (e.g., against hepatitis), and interferon, a substance with potential for treating viral infections. Many other medically useful substances are also genetically engineered, including factor VIII (a blood protein used in the treatment of hemophilia and tissue plasminogen activator (TPA), a substance that dissolves blood clots.
How genetic engineering is done
The main technique for mass-producing useful proteins by genetic engineering is called recombinant DNA technology. DNA is the genetic material in cells that controls the manufacture of different of proteins.
The first step is to identify a specific gene within the DNA of a cell that controls the manufacture of a particularly useful protein. This involves a number of highly sophisticated laboratory techniques. The next step is either to extract the gene from the cell or, if the exact chemical structure of the gene can be worked out, to synthesize it.
The final step is to introduce the gene into the DNA of a suitable recipient cell. Enzymes can be used to split the recipient cell's DNA at a certain site and so produce a gap into which the gene can be spliced (hence the term recombinant DNA).
The types of cells or organisms suitable for such genetic alteration are those that can subsequently be made to reproduce rapidly and indefinitely. The most popular organisms to date have been the common intestinal bacteria Escherichia coli and various yeasts, but cells of other organisms, including human cancer cells, have also been used with success.