Cloning was a significant breakthrough in molecular biology because it became possible to obtain homogeneous preparations of any desired DNA molecule
in amounts suitable for laboratory-scale experiments.
A single organism, the bacterium Escherichia coli,
played the dominant role in the early years of the
recombinant DNA era. This bacterium had always
been a popular model system for molecular geneticists and, prior to the development of recombinant
DNA technology, there were already a large number
of well-characterized mutants, gene regulation was
understood, and many plasmids had been isolated. It
is not surprising that the first cloning experiments
were undertaken in E. coliand that this organism
became the primary cloning host. Subsequently,
cloning techniques were extended to a range of
other microorganisms, such as Bacillus subtilis,
Pseudomonasspp., yeasts, and filamentous fungi, and
then to higher eukaryotes. Despite these advances,
E. coliremains the most widely used cloning host
even today because gene manipulation in this
bacterium is technically easier than in any other
organism. As a result, it is unusual for researchers to
clone DNA directly in other organisms. Rather, DNA
from the organism of choice is first manipulated in E.
coliand subsequently transferred back to the original
host or another organism, as appropriate. Without
the ability to clone and manipulate DNA in E. coli,
the application of recombinant DNA technology to
other organisms would be greatly hindered.