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chromosome walking

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The sequential isolation of clones carrying overlapping restriction fragments to span a segment of chromosome that is larger than can be carried in a phage or a cosmic vector. The technique is generally needed to isolate a locus of interest for which no probe is available but that is known to be linked to a gene that has been identified and cloned. This probe is used to screen a genome library. As a result, all fragments containing the marker gene can be selected and sequenced. The fragments are then aligned, and those segments farthest from the marker gene in both directions are subcloned for the next step. These probes are used to rescreen the genome library to select new collections of overlapping sequences. As the process is repeated, the nucleotide sequences of areas farther and farther away from the marker gene are identified, and eventually the locus of interest will be encountered. If a chromosomal aberration is available that shifts a particular gene that can serve as a molecular marker to another position on the chromosome or to another chromosome, then the chromosome walk can be shifted to another position in the genome. The use of chromosome aberrations in experiments of this type is referred to as chromosome jumping. See Chronology, 1978, Bender, Spierer, and Hogness.

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