) is lost. However, such information can be retrieved by a variety of methods which can identify clones with overlapping inserts.Chromosome walking means establishing clone contigs from fixed starting points.One widely used technique for identifying clones with overlappin inserts is to use a specific DNA probe from one clone to screen a DNA library. The positively hybridising clones should contain a DNA sequence that is closely related to the probe, including clones which contain sequences which partly overlap that found in the probe. This has often involved the preparation of a so-called end probe from the starting DNA clone: a fragment located at one end of the insert DNA and preferably present as a single copy of DNA, is purified and labelled. Positively hybridising clones can then be purified and new, distal end probes can be prepared for further rounds of hybridisation screening of the DNA library.YAC clone contig maps, as will be explained shortly, represented the first generation physical map of the human genome. A complete clone map of a chromosome would comprise all of the DNA without any gaps (contig originated as a shortened form of the word contiguous; sect 10.3. Because of their large inserts, yeast artificial chromosome (YAC) clones have been particularly useful in generating first generation physical maps of human chromosomes. Different methods of identifying overlaps between clones have been used, but STS markers (both polymorphic and non-polymorphic), which had previously been mapped to the chromosome of interest, have been particularly useful. Significant contig maps for individual human chromosomes were first reported in 1992 for chromosome 21 and the Y chromosome and, subsequently, a first-generation clone contig map of the human genome was reported by workers at the CEPH lab in Paris (Cohen et al, 1993). An updated YAC contig map, covering 75% of the human genome and consisting of 225 contigs with an average size of...
