Definition of Gene Splicing
Gene splicing is the procedure where pieces of DNA are cut up and spliced together to form recombinant DNA. The DNA fragments usually come from different organisms.
The DNA molecules are not cut by a physical pair of scissors; the process is enzymatic with the target DNA being chemically cut by restriction enzymes. There are thousands of these enzymes and each one recognises specific base pair sequences. They cut the DNA at the sites of recognition creating a broken DNA chain with a single strand exposed. Scientists can then add DNA sequences (i.e. a particular gene of interest) to this exposed strand using an enzyme called DNA ligase. When the DNA strand is repaired this recombinant DNA will create the protein product of the new gene.
Any form of DNA can be spliced together, no matter what its origin. For example, human insulin-producing genes have been spliced into plasmids. These then infect bacteria which multiply and produce bountiful supplies of insulin for diabetics. Previously the hormone had come from the pancreas of cadavers.
Gene Splicing History
Gene splicing came to the fore in 1973 when Herb Boyer and Stanley Cohen produced the first recombinant DNA organism by incorporating a gene from the African clawed toad Xenopus into bacterial DNA. But it was Stanford University scientist Paul Berg who a year earlier had used gene splicing to create the first recombinant DNA molecules.
Berg used restriction enzymes to cut the DNA of a phage and the DNA of the SV40 monkey virus. Both of these pieces of DNA occur in closed loops and the cuts turned them into linear molecules. Berg then modified the ends of these molecules with phage exonuclease and terminal transferase before adding adenine nucleotides to one type of DNA and thymine nucleotides to the other. When he combined the two molecules in a test tube their ends joined to form a circular hybrid of phage and SV40 DNA. Berg was the first scientist to splice together segments of DNA from different organisms, and his work was published in a landmark paper in the Proceedings of the National Academy of Science in 1972.
His advances in genetic engineering were made possible by the previous discovery of restriction enzymes (chemical scissors) by Daniel Nathans and Hamilton Othanel Smith and the discovery of DNA ligase (chemical soldering iron) which had been isolated in 1967.
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