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Recombinant cloning usually refers to recombinant DNA techniques. This involves the combining of DNA sequences that would not naturally exist. These techniques are also sometimes called genetic engineering. Specific segments of DNA are isolated and combined in a smaller unit of DNA that will replicate and amplify the number of cloned DNA molecules.
The host organisms for recombinant cloning are frequently bacteria. The vehicle of DNA used for cloning is known as a vector, and is usually a virus or a plasmid — a circular piece of bacterial DNA that is outside of the bacterial chromosome. A plasmid for cloning will have an origin of replication so that it can replicate itself, a cloning site, and some sort of selectable marker, such as antibiotic resistance. This will ensure the selection and propagation of the cells containing the clone.
The cloning site has a specialized sequence that will be recognized by a particular restriction endonuclease — an enzyme that recognizes specific DNA sequences and cuts the nucleotides. The plasmid will first be cut, so that it is linear. Researchers generally try to use restriction endonucleases that leave "sticky ends" that overlap and will anneal the ends of the target gene with compatible sequences in the vector using ligase, thus restoring it to a circle. Once the gene has been cloned into the vector, the vector is introduced into its expression host, usually by a process known as transformation, and grown up in large quantities. The DNA can then be isolated and used in experiments.
Recombinant cloning has permitted the analysis of large quantities of molecules that are normally only transiently expressed in the cell, such as mRNAs and proteins. This has revolutionized the study of biology. There are many practical applications of recombinant cloning.
Proteins such as human growth hormone can be expressed in large amounts through recombinant cloning. Transgenic plants are being used in agriculture to prevent attack by insects and pathogens. Many foods have been genetically modified to improve them, and cows have been treated with bovine growth hormone produced by recombinant cloning to increase milk production. Gene therapy has been used in many therapies, including to cure a form of blindness. Animals are even being cloned, although the clones frequently suffer health problems.
Some uses of recombinant cloning are highly controversial. Many people are opposed to eating genetically modified foods or drinking milk with recombinant hormone in it. There is much concern that crops that have been genetically altered may spread their new genes into the local flora.
Some people are philosophically opposed to altering existing life forms. There is almost universal condemnation of the idea of human cloning. It remains to be seen whether these new technologies will reach their full practical potential, or whether society will limit their use.