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.
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