Once recombinant DNA molecules have been constructed in vitro, the desired sequence can be isolated. In some experiments hundreds of thousands of different DNA fragments may be produced, and the isolation of a particular sequence would seem to be an almost impossible task. It is a bit like looking for the proverbial needle in a haystack – with the added complication that the needle is made of the same material as the haystack! Fortunately the methods available provide a relatively simple way to isolate specific gene sequences.

Three things have to be done to isolate a gene from a collection of recombinant DNA sequences: (i) the individual recombinant molecules have to be physically separated from each other,(ii) the recombinant sequences have to be amplified to provide enough material for further analysis, and (iii) the specific fragment of interest has to be selected by some sort of sequence-dependent method. In this chapter I consider the first two of these requirements, which in essence represent the systems and techniques involved in genecloning. This is an essential part of most genetic manipulation programmes. Even if the desired result is a transgenic organism, the gene to be used must first be isolated and characterised, and therefore cloning systems are required. Methods for selecting specific sequences are described in Chapter 8.

The biology of gene cloning is concerned with the selection and use of a suitable carrier molecule or vector, and a living system or host in which the vector can be propagated.