Screening and Selecting Transformants

Now suppose that this vector carries a piece of foreign DNA that is inserted into the plasmid DNA in such a way that it does not affect either of the genes for antibiotic resistance. Selection for antibiotic resistance would not be enough to guarantee that the selectees also carry the foreign DNA, since the selection did not operate on traits derived from that DNA. One must now use a screen of one type or another to find the desired clones. 

This method is often used in screening gene libraries made from sheared genomic DNA or from cDNA. There is, however, the technical difficulty of deciding on the sequence to be used in the probe, if the cloned gene s sequence is not yet known. Fortunately, there are some ways around this problem. 

---

There is then the problem of separating transformed cells, or transformants, that have acquired the correct recombinant DNA from cells that have not. This leads to the processes of screening and selecting transformants that have the desired... 

---