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duced can be used to distinguish different strains of bacteria, and help pinpoint if a particular strain was the cause of a widespread disease outbreak, for example.

Restriction fragment length polymorphism (RFLP) analysis has been widely used for identification of individuals (humans and other species). In this technique, genomic DNA is isolated, digested with a restriction enzyme, separated by size in an agarose gel, then transferred to a membrane. The digested DNA on the membrane is allowed to bind to a radioac-tively or fluorescently labeled probe that targets specific sequences that are bracketed by restriction enzyme sites. The size of these fragments varies in different individuals, generating a "biological bar code" of restriction enzyme-digested DNA fragments, a pattern that is unique to each individual.

Restriction enzymes are likely to remain an important tool in modern genetics. The reproducibility of restriction enzyme digestion has made these enzymes critical components of many important recombinant DNA techniques. see also Biotechnology; Cloning Genes; Gel Electrophoresis; Mapping; Methylation; Nucleases; Polymorphisms; Recombinant DNA.

Patrick G. Guilfoile

Bibliography

Bloom, Mark V., Greg A. Freyer, and David A. Micklos. Laboratory DNA Science: An Introduction to Recombinant DNA Techniques and Methods of Genome Analysis. Menlo Park, CA: Addison-Wesley, 1996.

Cooper, Geoffrey. The Cell: A Molecular Approach. Washington, DC: ASM Press, 1997.

Kreuzer, Helen, and Adrianne Massey. Recombinant DNA and Biotechnology, 2nd ed. Washington, DC: ASM Press, 2000.

Lodish, Harvey, et al. Molecular Cell Biology, 4th ed. New York: W. H. Freeman, 2000.

Old, R. W., and S. B. Primrose. Principles of Gene Manipulation, 5th ed. London: Blackwell Scientific Publications, 1994.

Internet Resource

Roberts, Richard J., and Dana Macelis. Rebase. <http://rebase.neb.com>.

Retrovirus

Retroviruses are RNA-containing viruses that use the enzyme reverse transcriptase to copy their RNA into the DNA of a host cell. Retroviruses have been isolated from a variety of vertebrate species, including humans, other mammals, reptiles, and fish. The family Retroviridae includes such impor-pathogens disease- tant human pathogens as human immunodeficiency virus (HIV) and human causing organisms Tlymphotropic virus (HTLV), the causes of AIDS and adult T-cell leukemia oncogenes genes that respectively. The study of this virus family has led to the discovery of oncogenes, resulting in a quantum advance in the field of cancer genetics. Retroviruses are also valuable research tools in molecular biology and gene therapy.

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