What Is Biotechnology? - Biotechnology and Your Health: Pharmaceutical Applications - page 10

The Polymerase Chain Reaction
To prepare a gene for engineering, the scientist generally needs
many millions of identical copies of the gene. Originally, the only
way to get these copies was to start with a large number of cells or a
large piece of tissue, to use chemicals to extract and purify the DNA,
and then to treat the DNA with an RE to clip the desired gene.
Today, there is a much faster way to copy the same piece of DNA. In
the mid-1980s, American chemist Kary Mullis developed a method
that could make many copies of a stretch of DNA, even when the
scientist knew only the sequence of bases at either end of the strand.
This technique, called the polymerase chain reaction ( PCR ), which
won Mullis the Nobel Prize in Chemistry in 1993, has become
a routine tool used in just about every research, hospital, and
criminal evidence laboratory that works with DNA. It allows
usable amounts of identical DNA molecules to be produced
from a small sample, about the amount found in just a few cells.
Once the gene has been isolated, the next step is to join it to
a molecular “on-switch,” a sequence of DNA that will allow the
cell to use the gene to make the desired protein. The on-switch,
called a promoter , is matched to the type of cell that will be used
for production.
Delivering the Gene to Its New Home
The next challenge is to get the desired gene into the new cell. The
target cell may be a bacterium, a yeast cell, or a cell from an insect,
plant, or mammal. Scientists use delivery systems, called vectors ,
suited for the cell type, to get the combination of gene and promoter
(sometimes called a “cassette”) into the target cell, so that DNA will
be copied each time the cell divides. The most commonly used vector
to get a gene into bacteria is a plasmid , a small circular piece of DNA
that is copied every time the bacterium divides into two, though it
does not become part of the bacterial DNA. Plasmids have been
developed that work as vectors for yeast, plant, and mammal cells.