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Biological vector

In biology, a vector is a means to transmit genetic information or whole organisms (usually pathogens) between other organisms. Viruses, for example, are considered as vectors because they spread by introducing their genetic information into their hosts. Vectors are becoming more common and powerful tools of research as the human genome and genomes of other organisms are being extensivly studied.

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Vectors in Gene Therapy

Viruses attack their hosts to insert their genetic material into the genetic material of the host. This genetic material contains instructions to produce these viruses. The host cell will carry out these intructions and produce the viruses. This is how viruses spread, in general.

In addition to the instructions producing the components of the virus itself, viruses can carry additional genes containing instructions for creating other kinds of proteins. In theory, if we insert a gene that is missing from a patient in a virus, and infect that patient with the virus, the virus will spread the missing gene in all the cells of the patient. The missing gene is now replaced and the disease is cured. This technique is called gene therapy.

Two types of viruses are currently used as vectors in gene therapy: retroviruses and adenoviruses[?]. They differ in their mecahnisms of action and results.

Retroviruses

The genetic material in retroviruses is in the form of RNA molecules, while the genetic material of their hosts is in the form of DNA. When a retrovirus infect a host cell, it will introduce its RNA together with some enzymes into the cell. This RNA molecule from the retrovirus must be produce a DNA copy from its RNA molecule before it can be considered part of the genetic material of the host cell. The process of producing a DNA copy from an RNA molecule is termed reverse transcription[?]. It is carried out by one of the enzymes carried in the virus, called reverse transcriptase. After this DNA copy is produced and is free in the nucleus of the host cell, it must be incorporated into the genome of the host cell. That is, inserted into the large DNA molecules in the cell, or the chromosomes of the cell. This proces is done by another enzyme carried in the virus called integrase.

Now that the genetic material of the virus is incorporated and has become part of the genetic material of the host cell, we can say that the host cell is now modified to contain a new gene. When this host cell divides later, its descendants will all contain the new genes.

One of the problems of gene therapy using retroviruses is that the integrase enzyme can insert the genetic material of the virus in any arbitrary position in the genome of the host. If genetic material happens to be inserted in the middle of one of the original genes of the host cell, this gene will be disrupted. If the gene happens to be one regulating cell division, uncontrolled cell division can occur (i.e.: cancer).

Gene therapy trials to treat Severe Combined Immunodeficiency (SCID) were halted when leukemia was reported in several of the patiens.

Adenoviruses

Adenoviruses are viruses that carry their genetic material in the form of DNA. When these viruses infect a host cell, they intorduce there DNA molecule into the host. The genetic material of the adenoviruses is not is not incorporated into the host cells genetic material. The DNA molecule is left free in the nucleus of the host cell, and the intructions in this extra DNA molecule are transcribed[?] just like any other gene. The only difference is that these extra genes are not replicated when the cell is about to undergo cell division. So the descendants of that cell will not have the extra gene. This means that treatment with the adenovirus will require regular doses to add the missing gene everytime new cells are produced without the gene.

Transgenic Animals

Transgenic animals[?] are animals produced with externally introduced genes. Transgenic animals can be used in many fields and as models to test the effect of certain genes on health. They can be used to produce "enhanced" versions of an animal.

They can also be used as bioreactors[?]: animals that produce an extra substance we want. Imagine having a transgenic cow that is modified to produce insulin in large quantities in its milk. This insulin can then be purified from the cows milk and used in replacement therapy[?] for treating patiens with diabetes mellitus.



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