Detection of Plasmid by Colony Hybridization

This experiment introduces colony hybridization, a technique central to genetic engineering technology. Why this technique is so important can be seen by considering the following problem. A scientific group wishes to clone in bacteria a particular human chromosomal gene. There would be no difficulty obtaining human DNA, cutting it with a restriction endonuclease, and ligating the many fragments obtained into vector molecules. This ligation mix could then be used to transform E. coli cells and plasmid-carrying colonies could be selected. The problem is that of identifying the clone carrying the required gene. Human chromosomal genes are most unlikely to be expressed in bacteria, and even less likely to give the colony a recognizable new phenotype. So how is it possible to select the one required clone from perhaps 100,000 others? By directly probing for the required DNA segment, colony hybridization provides a solution to this problem.