The knowledge gained through basic science research is fundamental to commercially valuable applications. For example, by understanding how genes are transcribed and translated, scientists can develop methods to suppress expression of certain genes.We know that only one strand of DNA is transcribed into a single strand of mRNA.This mRNA, the sense strand or plus (+) strand, is translated into a sequence of amino acids. An RNA molecule that is the complement of the sense strand is called an antisense strand or a minus (-) strand. Antisense RNA, which is not typically made by a cell, can base-pair with the sense strand to form a double-stranded RNA molecule, which cannot be translated.

Short fragments of antisense RNA can be chemically synthesized and used to interfere with gene expression. Recently, the first therapeutic drug based on antisense technology, fomivirsen, was approved for treating eye infections by cytomegalovirus (CMV) in AIDS patients. Fomivirsen is antisense RNA that is complementary to the mRNA of CMV; it prevents expression of two proteins required for viral replication. ■ cytomegalovirus, p. 757

Cells can also be genetically engineered to produce antisense RNA by introducing a copy of the gene with the promoter upstream from the antisense strand rather than from the sense strand (figure 1). Exploiting this principle, a plant biotechnology company genetically engineered tomato plants to synthesize antisense RNA of the gene that codes for the enzyme polygalacturonase.This plant enzyme breaks down plant cell walls and is responsible for the mushiness of ripe tomatoes a few days after they are picked. As a result of the genetic engineering, the tomatoes with antisense RNA to polygalacturonase do not get mushy for several weeks after they are picked, since the antisense RNA prevents polygalacturonase from being synthesized. Such technological achievement, however, does not guarantee economic success; commercially, the bioengineered tomatoes were a failure. ■ genetic engineering, pp. 220, 230

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