Contig maps thus consist of a linked library of small overlapping clones representing a complete chromosomal segment. DNA probe techniques can be used to fill in the gaps, but they are time consuming. Technological improvements now make possible the cloning of large DNA pieces, using artificially constructed chromosome vectors that carry human DNA fragments as large as 1 Mb. A more detailed map of a large YAC insert can be produced by subcloning, a process in which fragments of the original insert are cloned into smaller- insert vectors. Cloning involves the use of recombinant DNA technology to propagate DNA fragments inside a foreign host.
Vectors are DNA molecules originating from viruses, bacteria, and yeast cells.
Bacteria are most often the hosts for these inserts, but yeast and mammalian cells are also used. Cloning procedures provide unlimited material for experimental study. Genomic libraries are sets of overlapping fragments encompassing an entire genome. Also available are chromosome-specific libraries, which consist of fragments derived from source DNA enriched for a particular chromosome. PCR is especially valuable because the reaction is highly specific, easily automated, and capable of amplifying minute amounts of sample.