A vector is a vehicle by which foreign DNA is transferred from one cell to another. Some examples of vectors include modified viruses and plasmids. Viruses as Vectors Viruses are excellent vectors, because they have gained, through long periods of evolution, the ability to avoid destruction by the human immune system, and have the capacity to get their own genetic material inside specific cells. To use a virus as a vector, the harmful sections of its DNA are replaced with the desired cDNA to be introduced in the cell. Then, we allow the virus to infect' our host cell and if all goes well, the cDNA enters the cell and will be used to make the desired protein.
Some viruses can produce their own DNA and incorporate it into the host cell's genome.
These RNA-based retroviruses are the most common viral vectors used in Gene Therapy , where genes with therapeutic value are inserted into the retroviruses which, upon infection, incorporate these into the genome of the recipient cell. In other words, the harmful parts of the viral genome that serve to produce more viral particles have been removed and replaced by a sequence that codes for the protein of interest. The way in which the human insulin cDNA is introduced into bacterial cells is through the use of a plasmid. A plasmid is simply a loop of DNA containing genes that can easily diffuse into and out of bacterial cells. Although plasmids occur naturally in certain bacteria, the plasmids used for the purpose of introducing and expressing a foreign gene into a cell have been altered to such an extent, that the sequences they contain are very different from the naturally occurring plasmids upon which they are based.