We describe improvements in techniquesand strategies used for making maps of the human ge- nome. The methods currently used are changing andevolving rapidly. Todays techniques can produce orderedarrays of DNA fragments and overlappingsets of DNAclones covering extensive genomic regions, but they arerelatively slow and tedious. Methods under developmentwill speed the process considerably. New developmentsinclude a range of applications of the polymerase chaireaction, enhanced procedures for high resolution in situhybridization, and improved methods for generating,
manipulating, and cloning large DNA fragments. Moredetailed genetic and physical maps will be useful for
finding genes, including those associated with human diseases,long before the complete DNA sequence of the hu- man genome is available.
NEW TECHNOLOGY FOR MAKING PHYSICAL MAPS
Restriction enzymes are used to generate DNA fragments in all mapping, cloning, and sequencing protocols.Unfortunately,most naturally occurring restriction enzymes cut
DNA so frequently that the number of resulting DNA fragmentsis too large to order readily for mapping. Furthermore,avariety of factors, particularly DNA methylation,makes restriction enzymes at times unusable. Recently,naturally occurring restriction enzymes that recognize 8-bpsequences and therefore cut genomic DNA less frequentlyhave been identified . These enzymes along with certainothers with 6-bp recognition sites share the fact that they recognize DNA containing one or two occurrences of the dinucleotide sequence CpG. This DNA motif is underrepresented in the human genome . It often occurs near ex- pressed genes and is usually methylated. Examples of these restriction enzymes include Not I and Mlu I, which recognize the sequences GGCGGCCC and ACGCGT, respectively.Experimentally, these enzymes produce large fragments of cut human genomic DNA that are useful for mappingstudies. Recently, new techniques have been described for cutting genomic DNA into large fragments.