Vectors: Vehicles for cloning
Another major component of a gene cloning experiment is a vector such as a plasmid. A vector serves as a vehicle to carry a foreign DNA sequence into a host cell. A vector must possess certain features:
1. It must contain an origin of replication (ori) so that it is independently able to replicate within the host. This implies that any foreign insert it carries is automatically replicated.
2. It should incorporate a selectable marker, a gene whose product can identify the host cells containing the vector. Selectable markers include genes conferring antibiotic resistance, enzymes such as β-galactosidase which can turn substrates blue in the vicinity of the host cell colony and gene expressing Green Fluorescent Protein (GFP) which cause host cells containing the vector to fluoresce when viewed under UV light.
3. The vector must also have one unique restriction enzyme recognition site which can be used for cutting and introducing an insert. Most of the commonly used cloning vectors have more than one restriction site, they contain a Multiple Cloning Site (MCS) or polylinker. The MCS provides flexibility in the choice and use of restriction enzymes.
4. Another desirable feature of a cloning vector is that it should be small in size thereby facilitating entry/transfer into a host cell.
A number of vectors have been developed incorporating these features but only plasmids and bacteriophage based vectors will be discussed.