ABSTRACT

If the chromosome walk breaks down, perhaps because the library does not contain a suitable sequence, it is possible to use a technique called chromosome jumping to overcome this problem. Chromosome jumping can also be used when the ‘walk’ distance is quite large, for example when positional cloning from a relatively distant start point. Opposite ends of a long DNA fragment come together when the DNA is circularized or when cloned into a cosmid vector (see Topic H3). Without mapping the entire DNA, the end fragments can be (subcloned and) used as successive probes to screen a library and this can allow the isolation of clones that are around 50 kb from each other, so producing a more rapid ‘walk’ than the method shown in Fig. 2.