The first technique of genetic engineering, the plasmid method,
is the most familiar technique of the three, and is generally used
for altering microorganisms such as bacteria. In the plasmid
method, a small ring of DNA called a plasmid
(generally found in bacteria) is placed in a container with special
restriction enzymes that cut the DNA at a certain
recognizable sequence. The same enzyme is then used to treat the
DNA sequence to be engineered into the bacteria; this procedure
creates "sticky ends" that will fuse together if given the
opportunity.
Next, the two separate cut-up DNA sequences are introduced into
the same container, where the sticky ends allow them to fuse, thus
forming a ring of DNA with additional content. new enzymes are
added to help cement the new linkages, and the culture is then
separated by molecular weight. Those molecules that weigh the most
have successfully incorporated the new DNA, and they are to be
preserved.
The next step involves adding the newly formed plasmids to a
culture of live bacteria with known genomes, some of which will
take up the free-floating plasmids and begin to express them. In
general, the DNA introduced into the plasmid will include not only
instructions for making a protein, but also antibiotic-resistance
genes. These resistance genes can then be used to separate the
bacteria which have taken up the plasmid from those that have not.
The scientist simply adds the appropriate antibiotic, and the
survivors are virtually guaranteed (barring spontaneous mutations)
to possess the new genes.