Prokaryotic Cloning Vectors

The plasmid acts as a cloning vector, providing the replicative ability that enables the cloned gene to be propagated inside the host cell. Plasmids replicate efficiently in bacterial hosts because each plasmid possesses an origin of replication which is recognized by the DNA polymerases and other proteins that normally replicate the bacterium's chromosomes. The host cell's replicative machinery therefore propagates the plasmid, plus any new genes that have been inserted into it. Plasmids are uncommon in eukaryotes, although Saccharomyces cerevisiae possesses one that is sometimes used for cloning purposes; most eukaryotic vectors are therefore based on virus genomes. Alternatively, with a eukaryotic host the replication requirement can be bypassed by performing the experiment in such a way that the DNA to be cloned becomes inserted into one of the host chromosomes. Bacteriophage genomes can also be used as cloning vectors because they too possess origins of replication that enable them to be propagated inside bacteria, either by the host enzymes or by DNA polymerases and other proteins specified by phage genes.

 

Figure :An outline of gene cloning.

(Notes from : Gene Cloning and DNA Analysis- TA Brown 6^th Edition)