Recombinant DNA technology has made it possible to test gene function in bacteria or cell cultures rather than animal models.

Because of the universality of the genetic code, the polymerases of one organism can accurately transcribe a gene from another organism. For example, different species of bacteria obtain antibiotic resistance genes by exchanging small chromosomes called plasmids. In the early 1970s, researchers in California used this type of gene exchange to move a "recombinant" DNA molecule between two different species. By the early 1980s, other scientists adapted the technique and spliced a human gene into E. coli to make recombinant human insulin and growth hormone. Recombinant DNA technology — genetic engineering — has made it possible to gain insight into how genes work. In cases where it is impractical to test gene function using animal models, genes can first be expressed in bacteria or cell cultures. Similarly, the phenotypes of gene mutations and the efficacy of drugs and other agents can be tested using recombinant systems.

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