plasmids and bacteriophages have the ability to replicate within bacterial cells independent of the control of chromosomal DNA.
Bacteriophages because of their high number per cell, have very high copy numbers of their genome within the bacterial cells. Some plasmids may have only one or two copies per cell whereas others may have 15-100 copies per cell. Their numbers can go even higher. If we are able to link an alien piece of DNA with bacteriophage or plasmid DNA, we can multiply its numbers equal to the copy number of the plasmid or bacteriophage. Vectors used at present, are engineered in such a way that they help easy linking of foreign DNA and selection of recombinants from non-recombinants.

The following are the features that are required to facilitate cloning
into a vector.

# (i) Origin of replication (ori) : This is a sequence from where replication starts and any piece of DNA when linked to this sequence
can be made to replicate within the host cells. This sequence is also responsible for controlling the copy number of the linked DNA. So,
if one wants to recover many copies of the target DNA it should be cloned in a vector whose origin support high copy number.