pect is that it changes the forms of life we know of, possibly damaging our environment It has been known for some time that genetic information can be transferred between micro-organisms. This is process it done via plasmids (small circular rings of DNA) or phages (bacterial viruses). Both of these are termed vectors, this is because of their ability to move genetic material. In general this is limited to simpler species of bacteria. nevertheless, this can restriction can be overcome with the use of genetic engineering because it allows the introduction of any gene. While genetic engineering is beginning to be used to produce enzymes, the technology itself also depends on the harnessing of enzymes, which are available in nature. In the early 1970s Herbert Boyer, working at the University of California Health Science Centre in San Francisco, and Stanley Cohen at Stanford University found that it was possible to insert into bacteria genes they had removed from other bacteria. First they learned the trick of breaking down the DNA of a donor organism into manageable fragments. Second, they discovered how to place such genes into a vector, which they used to ferry the fragments of DNA into recipient bacteria. Once inside its new host, a transported gene divided as the cell divided, leading to a clone of cells, each containing exact copies of the gene. This technique became known as gene cloning, and was followed by the selection of recipient cells containing the desired gene. The enzymes used for cleaving out the DNA pieces act in a highly specific way. Genes can, therefore, be removed and transferred from one organism to another with extraordinary precision. Such manoeuvres contrast sharply with the much less predictable gene transfers that occur in nature. By mobilising pieces of DNA in this way (including copies of human genes), genetic engineers are now fabricating genetically modified microbes for a wide range of applications in industry, medicine and agriculture. The underlying idea of transferring genes between cells is quickly explained. However the actual practice is an extremely complicated process. The scale of the problem can be gauged from the astronomical numbers involved: the DNA of even the simplest bacterium contains 4,800,00 pairs of bases. But there is only one copy of each gene in each cell. First, restriction enzymes are used to snip the DNA into smaller pieces, each containing one or just a few genes. These enzymes cut DNA in very precise ways. They recognise particular stretches of bases (termed recognition sequences) and snip each strand of the double helix at a particular place. Whenever the recognition sequence appears in the long DNA chain, the enzyme makes a cut. Whenever the same enzymes are used to break up a certain piece of DNA, they always produce the same set of fragments. The cuts produce pieces of double helix with short stretches of single stranded DNA at each end. These are know as sticky ends. If the enzyme is allowed to act for a limited time, it may not have a chance to attack all the recognition sequences in the chain. This will result in longer fragments. As in natural DNA replication, bases have an inherent propensity to join up with their partners A with T, for example, and G with C. So too with sticky ends. For example, the sequence TTAA will tend to re-associate with AATT. Genetic engineers use another type of enzyme, DNA ligase, to make the union permanent. This is the key principle of genetic engineering the use of two types of enzyme to cut out one piece of DNA and then to attach it to another piece. The genetic engineer's toolkit now contains several hundred different restriction enzymes. Each is a precision instrument for fragmenting DNA in a particular way. Some recognise different base sequences; others recognise the same sequence but snip at a different point within or next to the sequence. Ferrying DNA to a new home Once a piece of DNA has been broken up into a mixture of fra

Some common words found in the essay are:
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Approximate Word count = 2940
Approximate Pages = 12 (250 words per page double spaced)