Biotechnology and Recombinant DNA

■ n September of 1976, Argentinean newspapers reported a

■ violent shootout that had occurred between soldiers and the JL. occupants of a house in suburban Buenos Aires, leaving the five "extremists" inside dead. Conspicuously absent from those reports was the identity of the "extremists"—a young couple and their three children, ages six years, five years, and six months. Over the next seven years, similar scenarios recurred as the military junta that ruled Argentina eliminated thousands of its citizens who were perceived as threats. This "Dirty War," as it came to be known, finally ended in 1983 with the collapse of the military junta and the election of a democratic government. The new leaders opened previously sealed records, which confirmed what many had already suspected—that more than 200 children had survived the carnage and had in fact been kidnapped and placed with families in favor with the junta.

Dr. Mary-Claire King was at the University of California at Berkeley when her help was enlisted in the effort to return the kidnapped children to the surviving members of their biological families. Dr. King and others recognized that the rapidly developing methods of DNA technology could be put to use for this important humanitarian cause. DNA is composed of a string of nucleotides, the sequence of which can serve as a molecular "fingerprint" of an individual. By analyzing certain DNA sequences, blood and tissue samples from one individual can be distinguished from those of another. These same principles can also be used to show that a particular child is the progeny of a given set of parents. Because a person has two copies of each chromosome—one inherited from each parent—one half of a child's DNA will represent maternal sequences and the other half will represent paternal traits. The case of the Argentinean children presented a great challenge, however, as most of the parents were dead or missing. Often, the only surviving relatives were aunts and grandmothers and it is difficult to use chromosomal DNA to show genetic relatedness between a child and such relatives. Dr. King decided to investigate mitochondrial DNA (mtDNA). This organelle DNA, unlike chromosomal DNA, is inherited only from the mother. A child will have the same nucleotide sequence of mitochondrial DNA as his or her siblings, the mother and her siblings, as well as the maternal grandmother.

By comparing the nucleotide sequences of mitochondrial DNA in different individuals, Dr. King was able to locate key positions that varied extensively among unrelated people, but were similar in maternal relatives. Dr. King's technique, born out of a desire to help reunite families victimized by war, has now found many uses. Today her lab, which is now at the University of Washington, remains very active using molecular biology techniques for humanitarian efforts, identifying the remains of victims of atrocities around the world.

—A Glimpse of History

A REVOLUTION HAS OCCURRED IN MOLECULAR biology over the past several decades—the science has been transformed from a descriptive study of what cells are, to an intricate study of how cells function. A driving force in that revolution was the development of simple methods to extract and manipulate DNA, the blueprint of life.

Biotechnology is the use of microbiological and biochemical techniques to solve practical problems and produce more useful products. In the past, this usually meant laboriously searching for naturally occurring mutants that produced maximal product or expressed other desirable characteristics. Today, the rapid developments in recombinant DNA techniques, the methods scientists use to study and manipulate DNA, have made it possible to genetically alter organisms to give them more useful traits. Ushering in the era of recombinant DNA technology was the discovery of restriction enzymes. These naturally occurring enzymes act as molecular scissors that recognize and cleave specific sequences of DNA;

220 Chapter 9 Biotechnology and Recombinant DNA

the sequence recognized by a particular restriction enzyme is usually four or six base pairs in length and is called the recognition sequence. The enzymes are remarkable tools for molecular biologists because they cut DNA in a predictable and controllable manner. Researchers can isolate genes from one organism, use restriction enzymes and other components of biological systems to manipulate the purified DNA in vitro, and then transfer the genes into another organism, a process called gene cloning. The more that scientists learn about genes and their functions, the more the field of biotechnology advances. In fact, biotechnology is now nearly synonymous with genetic engineering, the process of deliberately altering an organism's genetic information. ■ restriction enzymes, p. 231

Since the discovery of restriction enzymes, a virtual toolbox of DNA technologies has been developed, including methods to locate certain sequences in complex mixtures (nucleic acid hybridization), selectively replicate certain sequences (poly-merase chain reaction) and determine the nucleotide sequence of DNA (DNA sequencing). The information and innovations generated via these technologies are impacting society in innumerable ways—from agricultural practices and medical diagnoses, to evidence used in the courtroom.

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