To identify and amplify large quantities of cloned DNA, the ligated vector has
to be transformed into a bacterial host. Most methods for bacterial
transformation are based on an observation of MANDEL and
HIGA[113], who demonstrated that uptake of
bacteriophage 
DNA is enhanced by treatment of bacterial cells with
calcium chloride. Many variations in this basic technique have been
described, all directed towards optimizing the efficiency of transformation for
different bacterial strains. Most protocols yield 10 
- 10 
transformants
per microgram of intact pBR322 DNA.
Impressive though this efficiency is, it is important to realize, first, that
only a very small proportion of the cells are competent to incorporate plasmid
DNA in a stable fashion, and second, that only 1 DNA molecule in approximately
10,000 is successful at transformation.
Once inside the bacterium, the plasmid DNA replicates and expresses the
drug-resistance markers that allow the transformed cells to survive in the
presence of an antibiotic.
The ability of bacteria to take up DNA is short-lived. After exposure to agents that enhance uptake, most strains of bacteria remain in a competent state for only 1-2 days.
Cells