Gene insertion, the incorporation of exogenous genetic material into a genome, can occur naturally or can be artificially induced.
In nature, mobile elements called insertion sequences exist. They encode only the information necessary for their insertion into DNA. Depending upon the insertion sequence, they can insert at specific regions or at random. As they do not carry other genes, they tend not to be used as tools of genetic research. However, since both their insertion and exit from DNA can be disruptive, leading to the development of mutations, knowledge of their behavior is relevant to researchers. One of the disruptions, insertional duplication, is the duplication of a short region of the insertion sequence flanking the sequence itself. If the insertion sequence subsequently exits the genome, this duplicated area may be left behind, which can be disruptive if the host region of DNA codes for a protein. Another disruption to host DNA is the deletion of some of the host sequences upon integration of the insertion sequence.
Genes can also be inserted via the use of insertion vectors. For plants, the most common vector is a bacterium called Agrobacterium tumefacsiens. The bacterium has the ability to incorporate DNA into plant genetic material, which causes a benign tumor. In genetic engineering, the desired gene is hooked onto the A. tumefacsiens DNA. When the recipient plant cells are exposed to the bacterium transfer of DNA, including the sequence of interest occurs. Viruses also are gene vectors. The viral infection process involves the integration of viral genetic material into the host genome. Viruses can thus be engineered to deliver DNA, which has been introduced into the viral genome prior to infection. Gene insertion can also be achieved mechanically by microinjection of the gene into a cell, or ballistically, by shooting gold beads coated with the gene of interest into the cell.
The insertion of the genetic material is a random event, with no control yet possible for the routine insertion of DNA at precise locations. For this reason, large numbers of cells must be screened in order to identify an insertion event at a desired location. For example, the technique of insertional mutagenesis relies upon such screening to detect the functional disruptions in the host cell caused by insertional disruption of the gene encoding the function.
Research is ongoing into the use of introns to deliver genes to the host DNA more precisely. Introns, which are capable of inserting themselves into DNA, do so by recognizing specific sequences. Modifying both the intron and the target sequence can also change the site of insertion. This raises the possibility that introns may be useful in gene therapy, with the delivery of DNA to correct the defect in a gene.
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