This is the process of introducing the recombined
plasmids into the host cells, which is generally E. coli.
The mixture of DNA molecules obtained in step 2 will
then be added to an E-coli mixture. They are made competent, that
is, able to take up DNA from the solution, by addition of calcium
ions in the form of calcium chloride. The E. coli cells takes
in the plasmids by endocytosis.
Only about 1% will take up DNA from the solution and
only a small proportion of these transformed cells will contain
the required recombinant plasmids.