Bacterial Artificial Chromosome (BAC)

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Bacterial artificial chromosome (BAC)

Bacterial artificial chromosomes (BACs) involve a cloning system that is derived from a particular plasmid found in the bacterium Escherichia coli . The use of the BAC allows large pieces of deoxyribonucleic acid (DNA ) from bacterial or non-bacterial sources to be expressed in Escherichia coli. Repeated expression of the foreign DNA produces many copies in the bacterial cells, providing enough material for analysis of the sequence of the DNA. BACs proved useful in the sequencing of the human genome.

The BAC is based on a plasmid in Escherichia coli that is termed the F (for fertility) plasmid. The F plasmid (or F factor) contains information that makes possible the process called conjugation . In conjugation, two Escherichia coli bacteria can physically connect and an exchange of DNA can occur.

A BAC contains the conjugation promoting genetic information as well as stretch of DNA that is destined for incorporation into the bacterium. The foreign DNA (e.g., portion of human genome) is flanked by sequences that mark the boundaries of the insert. The sequences are referred to as sequence tag connectors. When the BAC becomes incorporated into the genome of Escherichia coli the sequence tag connectors act as markers to identify the inserted foreign DNA.

Using a BAC, large stretches of DNA can be incorporated into the bacterial genome and subsequently replicated along with the bacterial DNA. In molecular biology terminology, pieces of DNA that contain hundreds of thousands of nucleotides (the building blocks of DNA) can be inserted into a bacterium at one time. As the process is done using different sections of the foreign DNA, the amount of DNA that can be analyzed can be very large.

BACs were developed in 1992. Since then, their usefulness has grown immensely. The primary reason for this popularity is the stability of the inserted DNA in the bacterial genome. Because the inserted DNA remains in the bacterial genome during repeated cycles of replication, the information is not lost. As well, the BAC can be sequenced using the normal tools of molecular biology.

The most dramatic recent example of the power of BACs is their use by The Institute for Genomic Research (TIGR ) in the technique of shotgun cloning that was employed in the sequence determination of the human genome. Many fragments of the human genome could be incorporated into BACs. The resulting "library" could be expressed in Escherichia coli and the sequences determined. Subsequently, these sequences could be reconstructed to produce the orderly sequence of the actual genome. This approach proved to be less expensive and quicker than the method known as directed sequencing, where a genome was sequenced in a linear fashion starting at one end of the genome.

The total number of fragments of the DNA from the human genome that have been expressed in Escherichia coli by the use of BACs is now close to one million. In addition to the human genome, BACs have also been used to sequence the genome of agriculturally important plants such as corn and rice, and of animals such as the mouse.

With the realization of the sequence of the human genome, the use of BACs is becoming important in the screening of the genome for genetic abnormalities. Indeed, BAC cloning kits are now available commercially for what is termed genomic profiling.

See also Biotechnology; Plasmid and plastid

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