(Cox et al., 1990). When a set of DNA markers from the human chromosome is assayed in a panel of such radiation hybrids, the patterns of cross-reactivity can be used to construct a map (fig 10.3). The principle being very similar to meiotic linkage analysis. However, the resolution achieved can be quite limited. Such maps, have however, been useful frameworks for mapping genes (transcription maps (13.3). Large-scale restriction maps have also been generated, such as the NotI restriction map of 21q (Ichikawa et al., 1993; fig 13.2). However, the most important maps are clone contig maps because these are the immediate templates for DNA sequencing.The construction of the ultimate physical map (the complete nucleotide sequence) requires considerable time and effort in the case of a very large DNA molecule such as that found in a chromosome. In order to provide a framework for this to be done efficiently, a series of cloned DNA fragments need to be assembled which collectively provide full representation of the sequence of interest. To ensure that there is complete representation, and no gaps, the series of clones should contain overlapping inserts forming a comprehensive clone contig (fig 10.13a). In principle, contig assembly is facilitated by the way in which genomic DNA libraries are constructed: as part of the strategy for maximising the representation of the library, the genomic DNA id deliberately subjected to partial digestion with a restriction endonuclease (by reducing the time of incubation and by using low concentrations of the enzyme). As a result, individual genomic DNA clones usually contain DNA sequences that partially overlap with the insert DNA of at least some of the other clones in the library (see fig 10.13b). The cloning step means that the individual DNA fragments are sorted into different cells and so the original positional information of the fragments (how they were related to each other on the original chromosomes...
