Artificial Chromosome Vectors and Genomic Libraries

Yeast artificial chromosome vectors act like real chromosomes in yeast and can store much longer DNA fragments, some over 150 kilobases in size, big enough for several genes along with their regulatory sequences. However, YAC vectors are difficult to manipulate, are prone to spontaneous rearrangement, and have been supplanted by bacterial artificial chromosome (BAC) vectors.

Escherichia coli BAC vectors are derived from the F plasmid of Escherichia coli. This coirimon bactenum of plasmid behaves like a chromosome and not like a typical plasmid. BACs the human gut, used in r /r r research as a model can store very large DNA fragments—in excess of three hundred kilobases organism in some cases, although typical fragments are about half that size. The unique features of BAC vectors are very well suited to creating and maintaining DNA libraries. For example, once a BAC vector enters a cell, it will exclude all other BAC vectors, which means that a given E. coli clone will contain only one unique library fragment. Furthermore, E. coli cells are relatively easy to grow and store, and DNA purification from the bacterium is straightforward. BAC libraries played a key role in the massive sequencing efforts that made up the Human Genome Project.

Many of the large-format DNA libraries (YACs, BACs) are used exclusively to store genomic DNA for sequencing projects. Larger fragments per-

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