The process of cloning, or producing exact genetic duplicates, is an
application of molecular genetics that continues to raise controversy. It has
recently become possible to clone not only DNA molecules, but also entire
complex organisms such as sheep. One method by which animal cloning may be
accomplished is demonstrated here. Cloning requires two types of cells: a
donor cell containing the complete genome of the animal of interest, and a
recipient unfertilized egg cell. In this case, sheep stem cells, which have a
unique capacity to become almost any type of body tissue, are used as donor
cells. The stem cells are collected from the sheep to be cloned and cultured
in vitro, or artificially, in the presence of a specially formulated growth
serum, shown as the green liquid. Then, cells to be used in cloning
experiments are grown at a reduced concentration of nutrients to induce
entrance into a state of dormancy, in which cell growth and division is
halted. It has been shown that, for cloning to work properly, both donor and
recipient cell must be at synchronized states in the cell cycle or in other
words, if the recipient is in a dormant state, the donor cell must also be
inactive. This induction into a state of non-division is represented here by
the removal of one cell from the nutrient media; this cell will act as the
DNA donor in this experiment. When the donor cells are ready for use, an
unfertilized egg cell is harvested from a sheep. Holding the cell in place
with a needle, an instrument called a micropipette is used to remove the
nucleus, and with it all of the cell’s DNA. As a result,
the egg enters a state of dormancy, which can then be inserted in place of
the nucleus. Usually an electric pulse, not shown here, is used to induce the
egg to fuse with the donor stem cell and accept its genetic material as a new
nucleus. Next, the cell is activated with protein media, shown as the green
liquid, to promote growth. The stem cell within the egg divides rapidly into
many cells; one is removed and again deactivated by growth in a serum that is
low in nutrients. The nucleus of a second unfertilized sheep egg is then
removed by the same technique as before, and the stem cell harvested from the
first egg is fused with the second egg. The fused cell is then activated with
protein media as before, and the cells again multiply rapidly. However, this
time no cells are removed and an embryo is allowed to develop. Finally, the
embryo is implanted into the womb of another sheep and allowed to develop
until birth – if all goes well, the result is a
healthy lamb that is genetically identical to the donor animal. It is
speculated that this technology may be applied to human beings as well as to
sheep, as an infertility treatment alternative. However, many of the clones
generated in laboratories at the present time have shown health problems such
as rapid aging and physical deformities. These issues, as well as ethical
concerns, have caused many people to object to cloning experiments involving
humans. Clearly, a great deal of thought, discussion, and refinement of
experimental techniques should occur before any attempt is made to clone a
human being.
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