Plasmids as Vectors
Plasmids are defined as autonomous elements, whose genomes exist in the cell as extrachromosomal units. They are self replicating circular (only rarely linear) duplex DNA molecules, which are maintained in a characteristic number of copies in a bacterial cell, yeast cell or even in organelles found in eukaryotic cells.

A foreign DNA segment can now be inserted, by joining the ends of broken circular DNA to the two ends of foreign DNA, thus regenerating a bigger circular DNA molecule that can now be separated by gel electrophoresis on the basis of its size.Selection of chimeric DNA is also facilitated by the resistance genes, which the plasmid may carry against one or more antibiotics.

If a plasmid has two such genes conferring resistance against two antibiotics and if the foreign DNA insertion site lies within one of these two genes, then the chimeric vector loses resistance against one antibiotic, the gene for which has foreign DNA inserted within its structure.