Bacterial artificial chromosomes (BACs) are plasmid-based vectors used by microbiologists to insert genes into bacteria. BACs can carry significantly more genetic material than other vectors, allowing for the study of larger genes. BACs are inserted into bacteria using electroporation and can be used to clone large quantities of genetic sequences for study. They have accelerated disease treatment research and enabled more effective production of sequences for genetic modification.
A bacterial artificial chromosome (BAC) is one of a class of tools, called vectors, that microbiologists use to insert genes into a bacterium, usually e coli. The insertion of genes alters the properties of the bacterium in a process called transformation. A scientist can alter a strain of bacteria using a BAC, then compare the altered bacteria to an unaltered strain to find out what role the inserted genes play in cell biology. While all vectors are used by scientists in a similar way, the BAC is notable for being able to carry significantly more genetic material than competing tools.
Over the years, scientists have developed a number of different types of vectors to modify the genetic makeup of bacteria. Most of these are created by modifying phages – viruses that only infect bacterial cells – or structures called plasmids. The bacterial artificial chromosome is one of several plasmid-based vectors. Plasmids are free-floating rings of DNA that many bacteria contain in addition to their chromosomal DNA. They are not considered a separate form of life, but still behave like an organism within an organism: they can reproduce independently of the bacteria they “live” in.
Plasmids such as the bacterial artificial chromosome are inserted into bacteria using a process called electroporation. Electroporation involves disturbing the cell membrane with an electric shock, which creates temporary openings through which molecules can be inserted. BAC precursors included modified plasmids with exotic names such as cosmid and phosmid. These research attempts often frustrated because they could only carry a few tens of thousands of base pairs of DNA, enough to insert only very small genes.
In 1992, the first bacterial artificial chromosome was created by Hiroaki Shizuya, a researcher at the California Institute of Technology, by modifying a plasmid called factor F. Factor F plasmids are used naturally by bacteria to transfer DNA from one cell to another during periods of environmental stress, in order to increase genetic variability and the probability of survival. Unlike its predecessors, the BAC could carry large genes with hundreds of thousands of DNA base pairs or multiple genes at once.
A number of large BAC libraries are now operated by universities, private industry and government groups. In addition to the genes under investigation, many BACs contain tools that allow for easier searching. For example, some BACs contain genes that turn bacteria blue or glow, for easier identification. Some contain genes that make the host resistant to certain antibodies. Cultures can be purified by washing them with the antibody in question, killing all bacteria except those carrying the BAC.
Because bacteria reproduce rapidly, the bacterial artificial chromosome can also be used to clone large quantities of a particular genetic sequence for study. This has enabled better study of the genomes of organisms that grow slowly or unpredictably under laboratory conditions. The ability to clone has accelerated disease treatment research by enabling faster identification of effective antiviral and antibacterial drugs. It has also enabled more effective production of sequences used in the genetic modification of other organisms, for research and industry.
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