Bioassay for Target Gene Effect

Once a gene has actually been inserted into a host genome, the next practical question is simply: what does it do? A 'target gene' - also called target DNA, foreign DNA, passenger DNA, exogenous DNA, the gene of interest, or insert DNA, all interchangeable names for the same thing in this context - is the specific piece of DNA that is either being cloned, or being deliberately mutated so its function can be studied by observing what changes when it stops working. Gene-targeting experiments that specifically disrupt a gene inside the nucleus are described as producing a 'gene knock-out'. Two kinds of targeting vector are used to actually make this happen. Insertion vectors start out circular but become linear during the insertion process itself, since they are linearised specifically within the region of homology to the target locus; a side-effect of this mechanism is that it duplicates the sequence immediately adjacent to the selectable marker. Replacement (transplacement) vectors work differently: the vector's homology region is co-linear with (matches the same order as) the target sequence, the vector is linearised outside that homology region before being introduced into the cell, and a crossing-over event then swaps the incoming DNA in for the corresponding stretch of the cell's own e …