Multiplication of Vector in Host - Whatever the method being used to introduce the recombinant molecule into E. coli, the number of cells that actually take up DNA will be relatively low. In the best conditions only 107 transformants are expected.

For the maximum propagation of the recombinant clones, the medium chosen for plating the cells should not allow the growth of non-transformed cells at all and secondly it should be able to help us in distinguishing the non-recombinants from the transformants.

If the cells are plated immediately after transformation, the yield of colonies will be low or we will get zero result. After the incubation period, the cells are plated on the medium for the selection. Normally the plates are plated such that every clone gets enough place to grow and form a characteristic colony.

For example, if the volume of cells after the incubation is 1 ml then around 10 plates are plated by taking 100 III or 0.1 ml of the solution on each plate. If the aim is to isolate a single clone, then the total volume can be plated on one or two plates. If the aim is to generate a genomic library, then the volume is plated in 25 plates or in large plates.

The other problem which is encountered during the multiplication of recombinant clones, is the diffusion of enzyme from non-recombinant clones to recombinant ones. This problem is normally encountered with vectors containing lacZ gene or blue-white selection as phenotypic marker.

When the plates are incubated for more than 36 hours, the p-galactosidase produced in the non-recombinant bacteria will diffuse to the area where the bacterial clone which is a recombinant one breaks the X-gal and stains the bacteria which is a recombinant one, thus giving false positive results.

Hence, the multiplication of transformants is an important step in gene cloning. Any mistake (during any of the step) in this process demands a heavy price either in terms of time or cost.