Vectors: Selection and Autonomous DNA Replication
Cloning a piece of DNA requires that it be replicated when it is put back into cells. Hence the DNA to be cloned must itself be an independent replicating unit, a replicon, or must be joined to a replicon. Additionally, since the efficiency of introduction of DNA into cells is well below 100%, cells that have taken up DNA, and are said to have been transformed, need to be readily identifiable. In fact, since only about one bacterial cell out of 105 is transformed, selections must usually be included to permit only the transformed cells to grow.
Vectors must fulfill the two requirements described above, replica-tion in the host cell and selection of the cells having received the transforming DNA. As mentioned earlier, two basic types of vectors are used: plasmids and phage. Plasmids contain bacterial replicons that can coexist with the normal cellular DNA and at least one selectable gene. Usually it is a gene conferring resistance to an antibiotic. Phage, of course, carry genes for replication of their DNA. Since DNA packaged in a phage coat can enter cells effectively, selectable genes on the phage usually are unnecessary.