Bacterial Artificial Chromosomes (BAC) Clones are used for high through put sequencing. This is a useful method for top-down sequencing of a already sequenced organism and for filling in contig gaps. BAC Clones are useful because they can host segments of DNA that are 150-350kbp's long.
Large segments of DNA are spliced using restriction enzymes, amplified and then implanted into the plasmid genome of the bacterial host. The segment is then sequenced and arranged by a computer program to replicate the entire genome sequence.
Wikipedia. Bacterial Artifical Chromosomes. Last modified 10/2/12. http://en.wikipedia.org/wiki/Bacterial_artificial_chromosome