The DNA of prokaryotic cells (bacteria) is relatively simple
in comparison to the DNA of eukaryotic cells. A bacterium has
about 1/1000 as much DNA as a eukaryotic cell. In addition to
chromosomal DNA, a bacterium may also carry an additional circular
piece of DNA called a plasmid. Plasmids are useful to bacterial
cells because they can carry genes (such as an antibiotic resistance
gene) which can allow the bacteria to grow in nonideal environments.
The plasmid has proven to be a useful tool for the molecular
biologist. Genes can be inserted into the plasmid which, in turn,
can be incorporated into a bacterium by a process called transformation.
Since the bacteria will rapidly multiply and create many copies
of the gene, the molecular biologist can retrieve relatively
large quantities of a protein produced by the bacteria or can
use this arrangement to study the function of a particular gene.
This unit will describe how to extract a plasmid from a bacterial
cell (E .coli). This procedure is called a plasmid isolation.
Isolated colonies of bacteria containing the ampicillin resistance
gene (designated Ampr) inserted into the plasmid will be aseptically
transferred to a bacterial growth medium, grown overnight, then
chemically treated to separate bacterial chromosomal DNA and
proteins from the plasmid DNA.