a.k.a. Top-down Mapping

    For this type of mapping, a single chromosome is cut by a restriction enzyme into large pieces. The restriction enzyme most likely will have a 6 or 8 base recognition site to ensure the pieces are large. The large pieces are then ordered and subdivided. The smaller pieces then continue to be further mapped. 

    The order of and distance between sites, at which the rare-cutter enzymes cleave, are then determined which results in the macrorestriction map. There is more continuity and fewer gaps between fragments than what is seen on a contig map. However, map resolution is lower and this map may not be useful for finding particular genes. The macro restriction mapping technique does not generally allow long stretches of mapped sites. This approach allows DNA pieces to be located in regions measured about 100,000bp to 1_Mb. 

     The mapping and cloning of large DNA pieces has been greatly improved by the development of pulsed-field electophoretic methods. Gel electrophoresis normally can only separate pieces less than 40kb (1kb = 1000 bases) in size. PFG (Pulse-field gel electrophoresis) can separate molecules up to 10mb. This allows both conventional and new mapping methods to larger genomic regions to be applied in top-down mapping.