The enzymes that are used in gene manipulation are described in Chapter 5. The discovery of restriction endonucleases, and the use of type II restriction enzymes in generating DNA fragments is outlined. The three types of fragment ends (blunt or flush-ended, 5’ protruding and 3’ protruding) enable DNA from different sources to be joined together, using the enzyme DNA ligase. DNA modifying enzymes include polymerases (DNA and RNA polymerases and reverse transcriptase), nucleases that act on the ends of molecules (exonucleases) or within a DNA strand (endonucleases), transferases, kinases and phosphatases. The enzymes make up the ‘toolkit’ that allows DNA to be manipulated in the test tube (in vitro) to generate recombinant molecules.

To introduce the subject, the history of genetics since Mendel’s work which was rediscovered in 1900 is outlined. The discovery of the structure of DNA in 1953 marked the start of the molecular genetics era. When restriction enzymes and DNA ligase were discovered, DNA fragments could be cut and joined, with the first recombinant DNA molecules generated in 1972. Rapid methods for sequencing DNA were developed in the late 1970s and eventually were improved to the level needed to enable the Human Genome Project to be undertaken. The completion of this in 2003 marked the start of the ‘post-genomic era’ that led to further development of the technology and a reduction in time and cost of genome sequencing. We are now firmly in the post-genomic era, where DNA technology is having a major impact in areas such as transgenic plants and animals, genome editing, diagnosis and treatment of disease, forensic analysis and personalised medicine.