What Is Biotechnology? - Biotechnology and Your Health: Pharmaceutical Applications - page 18

has been modified to carry a new, foreign gene is called a transgenic
animal . There are two ways to make transgenic mice. Both use a
cassette of the desired gene in an appropriate vector.
One method, first successfully attempted in mice in the 1970s,
uses newly fertilized eggs. The researchers use a very fine needle to
inject the desired DNA into the area containing the DNA from the
sperm, before the sperm and egg have fused. The injected DNA may
become part of the sperm DNA. After fusion has occurred, the
fertilized egg is allowed to develop into a two-cell embryo, which is
then implanted into a female mouse. If the embryo attaches to the
uterus , the pregnancy will go forward and healthy pups, or baby
mice, will develop. Successful development of transgenic mice is
not certain, because only one-third of the embryos placed into a
mouse uterus develop into live animals, and only a few may carry
the transgene and produce the desired protein.
A second method provides a more certain outcome. Before the
manipulated embryo is put into the mouse's uterus, that genetic
information is present and in a form that will drive production of
a protein. This method uses embryonic stem ( ES ) cells . Early in its
development, before it settles in the uterus, the embryo becomes a
hollow ball of cells called the blastocyst , and inside that ball are
embryonic stem (ES) cells (Figure 1.6). A single mouse ES cell can
develop into a whole animal. Several research groups have reported
that an entire mouse was produced from an ES cell that was
allowed to grow and develop into an embryo in the lab and was
then placed in the uterus of a female mouse to complete its devel-
opment. To make a transgenic animal using ES cells, the researcher
first constructs the DNA cassette containing the desired gene, the
gene that will signal whether the transformation succeeds, and the
appropriate stretches of DNA that tell the cell to read the genes and
make the proteins. All of this material is incorporated into a vector.
The researcher mixes the vector with the ES cells. Some ES cells will
take the new DNA into their genetic material. Then the ES cells are