Introduction

Since the early 1970s it has become possible to isolate DNA and fragment it in a highly specific manner into pieces comparable in size to that of a gene, and then by joining such pieces to a suitable vector, which is normally viral DNA or a plasmid, to insert the chimaeras so formed into another species or organism. Over the last decade this technology, generally referred to as genetic engineering, has become refined and many of the initial technical difficulties overcome. This technology triggered what is generally regarded as the third rapid advance in the understanding of genetics (the first being the rediscovery of Mendel's work in 1901, and the second the advent of molecular genetics which began about 1940). It is now potentially possible to isolate a gene and transfer it either into another individual of the same species or into a different species. Further, by transferring it to a host with a short generation time, e.g. a bacterium such as E. coli with a generation time as short as 20 min, multiple copies or clones of the single gene may be produced. The much greater number of gene copies then available for isolation means that detailed structural studies can be carried out on the gene, including the determination of the sequence of its nucleotides.