In producing a recombinant plasmid to be used to clone a given donor insert, it is possible to cut both the donor and plasmid with the same restriction enzyme, resulting in complementary sticky ends. Assuming plenty of plasmid DNA is available, why is further selection necessary before the introduction of the plasmid into a cellular system?
1) Can you use the same restriction enzyme on the donor and the plasmid?
Sure, so long as the restriction sites are available and your enzyme doesn't create blunt ends. It is definitely possible.
2) Why do you need to do further selection before putting your recombinant plasmid into a cell?
This is a bit unclear because I'm not sure what selection you're talking about. If you're talking about doing selection on your DNA, this is probably referring to the fact that your recombinant plasmid has a lot of junk in the mixture from failed ligations. So it would improve your transformation efficiency to select only for your recombinant plasmid by, say, gel electrophoresis. If you're talking about selecting cells, you do have to prep them to be transformed, but this usually isn't done through 'selection'. You treat them chemically to become competent, or shock them.
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