1. Genetic engineering is the term applied to a set of techniques that allow scientists to alter the structure of genes to test hypotheses about how genes function or to allow expression of novel products. In the popular mind "genetic engineering" has the connotation of the generation of new forms of life. In reality the changes made in genetic engineering are usually quite small, much more like the effect of mutation and very unlike the process of speciation that results in "new forms of life".
2. The first step toward genetic engineering came from studies of a system used by bacteria to protect themselves from infection by viruses. Bacteria modify their DNA and express enzymes that fragment DNA that is not similarly modified; invading viruses that are not modified are destroyed by this system. Since the system limits, or restricts viral infection it was termed a restriction system, and the enzymes that fragment DNA were called restriction enzymes.
3. Hamilton Smith and Daniel Nathans developed a method to map the structure of DNA using the ability of restriction enzymes to fragment DNA by recognizing and cleaving small sequence motifs. Later, the teams of Alan Maxam & Walter Gilbert in the US and Alan Coulson & Fred Sanger in the UK developed competing methods to determine the sequence of nucleotides in DNA. The restriction and sequencing methodologies are the foundation of genetic engineering.
4. Recently, a technique invented by Kerry Mullis has revolutionized genetic engineering. This technique, termed polymerase chain reaction (PCR), uses specially designed DNA molecules to direct replication of a specified segment of a template DNA. The term "chain reaction" refers to the fact that the amount of DNA made increases exponentially as newly synthesized molecules act as templates for each successive round of replication. In principle, starting with only a single molecule of template DNA a PCR reaction can synthesize over 10 trillion molecules at the end of the reaction a few hours later. This method is now often used to create variants of DNA sequences for use in genetic engineering experiments.
5. Many scientists have used genetic engineering to introduce desirable traits into various species. Some have introduced genes encoding useful proteins. The earliest of these experiments expressed insulin in bacterial cells for use by diabetics. Others have introduced proteins that provide useful traits into agricultural species, like toxins active against insect pests into crop plants.

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1. Restriction enzymes provide the tools to create discrete DNA fragments.
2. Using bacterial plasmids to create "molecular clones".
3. The polymerase chain reaction allows rapid synthesis of novel DNAs.
4. Methods for the sequencing of nucleotides in DNA have led to the elucidation of the sequence of whole genomes, including the human genome.
5. Genetic engineering has been used for some useful purposes, though controversy continues.