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. For these reasons, PCR has also had a major impact on clinical medicine, genetic disease diagnostics, forensic science, and evolutionary biology. Repeated heating and cooling cycles multiply the target DNA exponentially, since each new double strand separates to become two templates for further synthesis. The two basic sequencing approaches, Maxam-Gilbert and Sanger, differ primarily in the way the nested DNA fragments are produced.