Vectors: Selection and Autonomous DNA Replication

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.