The Human Genome Project is expected to produce a sequence of DNA representing the functional blueprint and evolutionary history of the human species. However, only about 3% of this sequence is thought to specify the portions of our 50,000 to 100,000 genes that encode proteins. Thus an important part of basic and applied genomics is to identify and localize these genes in a process known as transcript mapping. When genes are expressed, their sequences are first converted into messenger RNA transcripts, which can be isolated in the form of complementary DNAs (cDNAs). Approximately half of all human genes had been sampled as of 15 June, 1996.
A small portion of each cDNA sequence is all that is needed to develop unique gene markers, known as sequence tagged sites or STSs, which can be detected in chromosomal DNA by assays based on the polymerase chain reaction (PCR). To construct a transcript map, cDNA sequences from a master catalog of human genes were distributed to mapping laboratories in North America, Europe, and Japan. These cDNAs were converted to STSs and their physical locations on chromosomes determined on one of two radiation hybrid (RH) panels or a yeast artificial chromosome (YAC) library containing human genomic DNA. This mapping data was integrated relative to the human genetic map and then cross-referenced to cytogenetic band maps of the chromosomes. (Further details are available in the accompanying article in the 25 October issue of SCIENCE).
The histograms reflect the distributions and densities of genes along the chromosomes. Because the individual genes (>16,000) are too numerous to represent, images have been chosen to illustrate the myriad aspects of human biology, pathology, and relationships with other organisms that can be revealed by analysis of genes and their protein products.
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