'Genetic engineering' or genetic manipulation as it should properly be called, relies essentially on the ability to manipulate molecules in vitro. Most biomolecules exist in low concentrations & as complex, mixed populations which it is not possible to work with effectively. This problem was solved in 1970 using the molecular biologist's favourite bug, Escherichia coli , a normally innocuous commensal occupant of the human gut. By inserting a piece of DNA of interest into a vector molecule, i.e. a molecule with a bacterial origin of replication, when the whole recombinant construction is introduced into a bacterial host cell, a large number of identical copies is produced. Together with the rapid growth of bacterial colonies all derived from a single original cell bearing the recombinant vector, in a short time (e.g. a few hours) a large amount of the DNA of interest is produced. This can be purified from contaminating bacterial DNA easily & the resulting product is said to have been 'cloned'.
Most vector molecules were originally derived from one of two sources:
Vector molecules & cloning are not the only contribution which microorganisms have made to genetic manipulation. The actual task of altering the DNA at a molecular level is carried out by the use of naturally-occurring enzymes - most of which are derived from bacteria or viruses:
EcoRI from Escherichia coli BamHI from Bacillus amyloliquefaciens
These systems operate by enzymes which recognise specific short regions of DNA sequence, which are usually palindromic ('Able was I ere I saw Elba'), e.g:
5' GGATCC 3' 3' CCTAGG 5'
Recently, thermostable polymerases have become important, e.g. Taq DNA polymerase from Thermus aquaticus. This bacterium has evolved to grow in hot springs at temperatures which kill most other species. These enzymes allow the amplification of as little as one molecule of DNA into a large amount by means of repeated cycles of melting, primer annealing & extension by the enzyme which is not destroyed by the high temperatures used in this process. This is known as the polymerase chain reaction:
The utility of cloning is partly analytical, i.e it provides the ability to determine the genetic organization of particular regions or whole genomes (the human genome will soon be underway). However, it also facilitates the production of naturally-occurring & artificially-modifed biological products by the expression of cloned genes. The ability to take a gene from one organism (e.g. man or a tree), clone it in E. coli & express it in another (e.g. a yeast) is dependent on the universality of the genetic code, i.e. the triplets of bases which encode amino acids in proteins:
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The Role of Microorganisms in Genetic Engineering