Gene therapy is a technique of genetics which uses genes to treat diseases. There are specific vectors used in this technique. Usually viruses are the microorganisms which play the role of vectors in gene therapy. The procedure of gene therapy is to insert the healthy gene in the genome which replaces the damaged gene. For this purpose, the genetically modified gene is inserted in the virus which carries this gene into the human body. They target the diseased gene and either they completely destroy it or block its activity depending on the information they are carrying. As it is known that viruses possess pathogenic features, scientists have taken advantage of this property and made use of them to remove the gene which is causing disease in the body.

Viral vectors:-
Following are the types of viruses which are used as vectors:-

This type of viruses has their DNA in the form of double strands. When the viruses cause any infection in the human body for example common cold, they insert the body and release their genetic material into the host organism's cell. The genetic material enters the nucleus, there it does not combine with the host cell's nucleus but remain as a separate DNA. There is one positive point about this virus is that, that when the host cell's genetic material replicates, viral DNA does not replicate that is why there are no chances that the disease will pass to the off spring genetically. This virus is used as a vector when the disease of cancer is treated with gene therapy. As the adenovirus does not incorporate with the host cell's DNA, that is why when the human cell's culture is made, there will be need of insertion of adenovirus in the new population of the cells.

Retroviruses have their genetic material in the form of RNA molecules. When these viruses attack the host cell, they transfer their genetic material in the form of RNA molecule. Some important enzymes also transfer with its genetic material into the host cell. As the host cell's genetic material is in the form of DNA molecules and viral genetic material is in the form of RNA molecules that is why RNA molecule has to make DNA molecule before it can combine with the DNA molecule of the host cell. For this purpose RNA molecule uses reverse transcriptase, an enzyme to make DNA molecule, which transcribes the RNA molecule into DNA molecule. After the development of DNA molecule, it combines with the genetic material of the host and starts reproducing. Unlike adenoviruses which do not pass their genetic material to the next generation cells, retroviruses pass their genetic material to the next generation cells. When the retroviruses incorporate their genetic material in the form of DNA molecule into the host cell's genetic material, another enzyme which is present in the retrovirus is used that is integrase.

When the retrovirus genetic material combines with the genetic material of the host cells then host cell's genetic material becomes modified as it contains new genes now. But retroviruses do not easily transfer their genetic information into the host cell's genetic material.

If the retrovirus is used in gene therapy, then there are some disadvantages in the sense that if the genetic material of virus is inserted in the middle of any gene, it will disrupt the gene and if the genetic material is inserted in the gene which is fixed for the cell division process, then there are bright chances that the cells will start dividing uncontrollably. It will result in the disease of cancer. But if gene therapy uses these viruses as vectors against the disease of Severe Combined Immunodeficiency, then they play significant role in curing this disease.

Herpes Simplex virus vectors:-
This type of virus is responsible for causing defects in the neurons of human brain. They are human neurotropic viruses. Though these viruses damage neurons but these neuron are not rejected by immune system but in some cases even damaged neurons function in a normal way.

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