The Role of Restriction Enzymes in Genetic Engineering
This is a writing sample from Scripted writer David M. Brown
Genetic engineering is any human-directed modification of the genetic makeup of organisms to change the traits of those organisms. The oldest technique of genetic engineering is selective breeding of farm animals. In recent decades, biologists have learned how to directly transfer genes from the DNA (deoxyribonucleic acid) of one organism to the DNA of others. The tool used to do this is nature's own "scissors": restriction enzymes. Function Restriction enzymes are enzymes (protein catalysts) used by bacteria to shred invading viruses. (Restriction enzymes do not attack the bacterium itself because its DNA has been chemically modified in a way that immunizes it.) Also called restriction endonucleases, restriction enzymes slice DNA at highly specific sites: at sections of the DNA consisting of short strings of the basic units (bases or nucleotides) of DNA. These stereotypic sequences are known as "recognition sequences." Potential Because the exact same recognition sequences exist in the DNA of different organisms, biologists can use restriction enzymes to cut a stretch of DNA from the genome of one organism and paste it into the genome of another. (A genome is any complete set of the DNA warehoused in a cell's nucleus.) Structure DNA consists of a double strand, the famed "double helix." Only four different kinds of bases or nucleotides make up DNA: adenine, guanine, thymine and cytosine (A, G, T and C). An A base always bonds with a T base to form a rung of the helix, and C always bonds with G. This complementary base pairing makes it possible to copy DNA during cell division and to export copied genetic information from the nucleus in the form of ribonucleic acid (RNA) for the purpose of making proteins. Results When a double strand of DNA is cut by a restriction enzyme, one or more bases typically stick out from one of the two strands at the point of the cut. This is a "sticky end" of the mostly double-stranded DNA segment that has been chopped out. Because the DNA of the recipient organism has been severed at the same sequence of nucleotides, the sticky ends of the recipient's exposed DNA can be matched with the sticky ends of the donor gene. This is possible largely because of complementary base pairing. Biologists use an enzyme called DNA ligase, one of the enzymes involved in copying DNA, to stitch the donated gene into the recipient DNA. Benefits The genetic engineering made possible by restriction enzymes yields a wide array of medical and other benefits. One medical benefit is faster and more abundant production of insulin, used to treat diabetes. The genomes of fast-reproducing bacteria are modified so that they incorporate the human gene for insulin production. Previously, insulin had to be taken from pigs and cows, a form of insulin production that was slow and costly—and less effective, since animal-harvested insulin does not function properly in every human patient. #
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