map, a physical map of the human genome will consist of 24 maps, one for each chromosome. The different genetic maps of the human genome thay have so far been assembled all represent the same concept –sets of linked polymorphic markers (linkage groups) corresponding to different chromosomes. However, unlike this uniformity, a variety of different physical maps are possible (table 13.3 ans figure 13.2) The first physical map of the human genome was obtained more than 40 years ago when cytogenetic banding techniques were used not only to distinguish between different chromosomes, but also to provide discrimination of different subchromosomal regions (figure 2.17). Although the resolution is coarse (an average size chromosome band in a 550-band preparation contains approximately 6 Mb of DNA), it has been very useful as a framework for ordering the locations of DNA sequences by chromosome in situ hybridisation techniques. This is a simple procedure for mapping genes and other DNA sequences by hybridising a suitably labelled DNA probe against chromosomal DNA that has been denatured in situ. To do this, an air-dried microscope preparation of metaphase or prometaphase chromosomes is made, usually from peripheral blood lymphocytes or lymphoblastoid cell lines. Treatment with Rnase and proteinase K results in partially purified chromosomal DNA, which is denatured by exposure to formamide. The denatured DNA is then available for in situ hybridisation with an added solution containg a labelled nucleic probe, overlaid with a cover slip. Depending on the particular technique used, chromosome banding of the chromosomes can be arranged either before or after the hybridisation step. As a result, the signal obtained after the removal of excess probe can be correlated with the chromosome pattern in order to indentify a map location for the DNA sequences recognised by the probe. Chromosome in situ hybridisation has been revolutionized by the use of ...
