These molecules of cDNA thus hybridize to "expressed" chromosomal regions -- regions that directly dictate the synthesis of proteins. However, a physical map that depended only on in situ hybridization would be a fairly coarse one. Fluorescent tags on intact chromosomes cannot be resolved into separate spots unless they are two to five million base pairs apart.
Fortunately, means are also available to produce physical maps of much higher resolution -- analogous to large-scale county maps that show every village and farm road, and indicate distances at a similar level of detail. Just such a detailed physical map is one that emerges from the use of restriction enzymes -- DNA-cleaving enzymes that serve as highly selective cleavers of the genetic material. A typical restriction enzyme known as @EcoRI, for example, recognizes the DNA sequence @GAATTC and selectively cuts the double helix at that site. One use of these handy tools involves cutting up a selected chromosome into small pieces, then cloning and ordering the resulting fragments. The cloning, or copying, process is a product of recombinant DNA technology, in which the natural reproductive machinery of a "host" organism -- a bacterium or a yeast, for example -- replicates a "parasitic" fragment of human DNA, thus producing the multiple copies needed for further study.
By cloning enough such fragments, each overlapping the next and together spanning long segments (or even the entire length) of the chromosome, workers can eventually produce an ordered library of clones. Each contiguous block of ordered clones is known as a contig, and the resulting map is a contig map. If a gene can be localized to a single fragment within a contig map, its physical location is thereby accurately pinned down. Further, these conveniently sized clones become resources for further studies by researchers around the world -- as well as the natural starting points for systematic sequencing efforts.
Once the DNA has been mapped, and all the genes placed within the chromosomal framework, the real work begins û making sense of the information encoded within those genes. Specifically, this involves determining whether the genes code for a particular protein, and if so, which one, or whether they are involved in the regulation of other genes. The task then falls to understanding what the proteins' functions are within the body, so that effective therapies can be developed for instances when they malfunction. While much progress has been made in the mapping of the human genome, it is clear to all involved that there is clearly much work yet to be done to gain a full understanding of the complex functioning of the human genetic material.
