Mapping the Genome 



The human genome may be "understood" at several levels (Fig. 10). Geneticists have 

 already charted the approximate positions of over 1000 genes, and a patchy start has 

 been made at establishing high-resolution maps of the genome. The coarsest maps are 

 chromosome and genetic maps, which define the chromosomal locations of genes. The 

 physical map is an ordered set of DNA fragments made from restriction enzyme 

 fragments. The ultimate map is the base pair sequence for the human genome. 



i-^' Libnin 

 III (linud 

 tragments^ ,5, 



Fig. 9. Linking a library of cloned 

 DNA fragments. Several strategies 

 are available for ordering an 

 unordered library of cloned restric- 

 tion fragments. The one schemati- 

 cally illustrated here involves 

 sequencing the ends of the 

 fragments, together with a corre- 

 sponding family of much shorter 

 linking clones. The linking clones, 

 themselves generated by restriction 

 enzymes, are fragments that 

 contain the cutting site (in this 

 simplified example. GGCC) for the 

 enzyme used to produce the clone 

 library. In practice, linking clones 

 are typically 50 to 100 base pairs 

 long, and the fragments being 

 linked are likely to be many 

 thousands of base pairs long. If the 

 library contains contiguous frag- 

 ments and if the family of linkers is 

 complete, assigning an order to the 

 fragments is a simple matter of 

 sequence matching — a puzzle- 

 solving task best done by computer. 



129 



