MicrobiologyBytes: Introduction to Microbiology: Genetic Engineering Updated: August 14, 2008 Search

The Role of Microorganisms in Genetic Engineering

'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:

Rapidly, the original vector molecules were greatly modified to improve their usefulness as vectors, e.g:

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:

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:

 

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Online PCR Experiment - Henrietta Lacks

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:

In fact, the genetic code is not completely universal - there are minor differences in the codons which are recognised & used by different groups of organisms, e.g. animals, plants & bacteria. However, this rarely presents a difficulty expressing foreign genes, & if it does, the means exist to modify particular codons to alternatives coding for the same amino acids which will be recognised by the host organism.

A list of some of the products produced by genetic engineering is given elsewhere, but this is expanding very rapidly & new products of great medical, agricultural, environmental & industrial importance are constantly being produced by this route.

The practical application of molecular biology is founded on our understanding of microorganisms. Microbiology is thus a central discipline in modern biology.


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by L.M.Prescott et al.
A balanced, comprehensive introduction to all major areas of microbiology. The sixth edition has been updated extensively to reflect the latest discoveries in the field.
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