Enzymes and recombinant DNA technology
Recombinant DNA technology, also called as genetic engineering makes use of a variety of enzymes, particularly restriction endonucleases (from bacteria) and DNA ligases to insert extra genes into cells with the help of vehicles termed 'vectors'. One important group of vectors is 'plasmids', which are small, circular, cytoplasmic molecules of DNA, acting as extrachromosomal genes in bacteria. It is possible to extract and purify plasmids and to insert extra genes into the circles. These altered plasmids can then be taken up again from the medium by the microorganism. Then as the bacterium is grown in culture, the inserted gene will be replicated together with the vector. The production of identical copies is termed as 'cloning.
Another group of vectors are the variants of a bacterial virus known as λ phage. The phage DNA is about 45 kb long, of which the middle third has no role in the infection process and can be replaced by another piece of DNA of about the same length (again by means of restriction endonuclease and ligase enzymes) without affecting the ability of the phage to infect bacteria and be reproduced in abundance. Genes or gene fragments of about 15 kb length can be inserted and cloned in this way, in contrast to the limit of 10 kb when plasmids are used as vectors.