Cloning Vectors for Recombinant DNA
One of the most important uses of recombinant DNA technology is the cloning of (i) random DNA or cDNA segments, often used as probes or (ii) specific genes, which may be either isolated from the genome or synthesized either organochemically or in the form of cDNA from mRNA. This cloning of DNA is possible only with the help of another DNA molecule, which is capable of replicating in the host. This other DNA molecule is often used in the form of a vector, which could be a plasmid, a bacteriophage, a derived cosmid (see later in this section) a phagemid (phage + plasmid) or even a virus. Techniques should also be available, which will allow selection of chimeric genomes obtained after insertion of foreign DNA from a mixture of chimeric and the original vector (see later). Another critical desired feature of any cloning vector is that it should possess a site at which foreign DNA can be inserted without disrupting any essential function. Therefore, in each case a restriction enzyme will also have to be selected which will cause a single break. Sometimes vectors are modified by inserting a DNA segment to create unique site(s) for one or more restriction enzymes to facilitate its use in gene cloning. This inserted DNA is sometimes called 'polylinker'.