Viral vectors are a tool commonly used by molecular biologists to deliver genetic material into cells. This process can be performed inside a living organism (''in vivo'') or in cell culture (''in vitro'').
Viruses have evolved specialized molecular mechanisms to efficiently transport their genomes inside the cells they infect. Delivery of genes by a virus is termed transduction and the infected cells are described as transduced.
Molecular biologists first harnessed this machinery in the 1970s. Paul Berg used a modified SV40 virus containing DNA from the bacteriophage lambda to infect monkey kidney cells maintained in culture.
Viral vectors are tailored to their specific applications but generally share a few key properties.
- ''Safety'': Although viral vectors are occasionally created
from pathogenic viruses, they are modified in such a way as to minimize
the risk of handling them. This usually involves the deletion of a part
of the viral genome critical for viral replication. Such a virus can
efficiently infect cells but, once the infection has taken place,
requires a helper virus to provide the missing proteins for production
of new virions.
- ''Low toxicity'': The viral vector should have a minimal effect on the physiology of the cell it infects.
- ''Stability'': Some viruses are genetically unstable and
can rapidly rearrange their genomes. This is detrimental to
predictability and reproducibility of the work conducted using a viral
vector and is avoided in their design.
- ''Cell type specificity'': Most viral vectors are
engineered to infect as wide a range of cell types as possible. However,
sometimes the opposite is preferred. The viral receptor can be modified
to target the virus to a specific kind of cell.
- ''Identification'': Viral vectors are often given certain
genes that help identify which cells took up the viral genes. These
genes are called Markers, a common marker is antibiotic resistance to a
certain antibiotic. The cells can then be isolated easily as those that
have not taken up the viral vector genes do not have antibiotic
resistance and so cannot grow in a culture with antibiotics present.
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