Traditionally in medicine, a vector is an organism that does not cause disease itself but which spreads infection by conveying pathogens from one host to another. Species of mosquito, for example, serve as vectors for the disease-causing West Nile Virus; which the insects may ingest by feeding from an infected bird and regurgitate into a human, infecting him or her. This sense of "biological vector" is the primary one in epidemiology and in common speech.
In gene therapy, a virus itself may serve as a vector, if it has been re-engineered and is used to deliver a gene to its target cell. A "vector" in this sense is a vehicle for delivering genetic material such as DNA to a cell.
Finally, in genetics more generally, DNA by itself may be regarded as a vector, for example in particular when it is used for cell transformation. A vector in this sense is a DNA construct, such as a plasmid or a bacterial artificial chromosome, that contains an origin of replication. An appropriate replication origin causes a cell to copy the construct along with the cell's chromosomes and to pass it along to its progeny. A single cell that has been transformed with a vector will grown into an entire culture of cells, which all contain the vector, as well as any gene attached to it within the construct. Because the constructs can be extracted from the cells by purification techniques, transformation with a vector is a way of making a small number of DNA molecules in to a much larger one.
There is a possibility for confusion between the use of "vector" in gene therapy and its use in molecular biology more generally. Some transformation technologies, such as lipofectamine, enable the direct delivery of a DNA construct as therapy in a tissue. In such a situation, a plasmid vector may be regarded as serving as its own gene-therapy vector. When a speaker calls it "a vector," they may be referring to either of its vector aspects or often both.
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3 Cell transformation and gene therapy