Gene Cloning

The Production and Uses of Transgenic Organisms 383
within them. No homology is needed at the new point where the transposon
inserts, and the newly inserted transposon may itself transpose again
at a subsequent time. Transposons can be adapted to use as powerful tools
for DNA manipulation and for generating mutations.
(a)
(b)
(c)
Since the production of the “supermice”, an alternative method has
been developed for making transgenic mice which utilizes microinjection
of embryonic stem cells (ES cells) rather than fertilized egg cells. ES cells
are cells isolated from embryos which can be cultivated in tissue culture.
They have the very important and useful property that, if injected back into
a developing embryo, they can go on to form cells of any cell type. If they
contain a transgene, the transgene will also be found in these cell types,
and if found in the cells of the germ line, the progeny will now contain the
transgene in all their cells. The use of ES cells is discussed further below,
when methods for making gene knockouts in mice are described.
Getting an animal from the transgenic cell
Once the fertilized egg has been successfully microinjected, it is transferred to
a “pseudo-pregnant” mouse. This is a female mouse which has been mated
with a vasectomized male, and which is hence receptive to the new fertilized
egg. The egg is transplanted into the oviduct, and if all goes well it will develop
into a fetus and eventually a baby mouse. There are many pitfalls in this technique:
the new DNA may not integrate into the chromatid, the egg may be
damaged by the microinjection process and fail to develop to term, or the
baby mice may be abnormal or defective in some way, due probably to