Science Watch
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- 2. Foreign Animal Species
- 3. The Road to Domestication
- 4. Demon Gene (Genetic technology)
Animal Cloning
Stephen L. Zawistowski, Ph.D., ASPCA
CloneCloneClone
On my bookshelf I have A Dictionary of Biology, 6th Edition, which was published in 1973. I bought it when I was a freshman biology major, and it more than repaid its $2.50 purchase price, as its well-thumbed pages offered quick answers as I prepared for numerous exams. It still retains a precious space on my bookshelf—not because I need a reference, but because it serves as a valuable reminder. A quick flip through its pages provides ample proof of the explosive growth that has occurred in the biological sciences over the past quarter century. It reminds me why I need to continue reading the new scientific journals that stack up on the corner of my desk each month. If anything rivals the accelerating pace of research, it is the ever-shrinking time lapse between discovery and front-page news.
My old dictionary, for example, provides the following definition of the headlining term, “clone.” It reads, “[Descendants produced] asexually or by parthenogenesis from a single animal; by division from a single cell.” Though accurate, this quaint definition says nothing about Dolly, Missy or myriad other topics the authors wouldn’t likely have anticipated less than 30 years later.
Of course, in some ways cloning isn’t new at all. Horticulturists have been avid practitioners for hundreds of years. Cutting a potato into fragments and planting them, or starting a new plant from a cutting, is a form of cloning. Each resulting new plant is genetically identical to the original. This basic-level cloning is possible because each plant cell nucleus contains the complete plant genome, or a full set of the genes needed to make that plant. Even though the “potato” is a part of the plant’s root structure, its cells contain the genes required to build stems and leaves, and to synthesize chlorophyll. The same is true of animal tissue cells. Each of your skin or muscle cells has the complete human genome—all of the genes needed to build a skeleton, nerves and blood. In other words, another you.
It’s All in the Technique
The reason that you can clone a potato with equipment no more sophisticated than a paring knife is because once the cells of the potato plant have differentiated, or become specialized, the cells of the potato retain the potential to differentiate once again, and become stems and leaves. Animal cells, once they have become muscle or nerve cells, no longer retain the ability to differentiate, even though they still have the genes to do so. To overcome this problem, a variety of sophisticated techniques are used.
When cloning a sheep, for example, scientists use hormones to stimulate a female to hyperovulate, or release more than the usual number of fertile eggs. These egg cells are collected and, with the help of a very fine micromanipulator, the nuclei are removed. Meanwhile, a nucleus is taken from the tissue cells of a donor sheep and injected into the egg cells. Chemical stimulation is then used to start the division process of the egg cell, which is kept in a culture dish until it has divided into 16 cells. At that point, it is implanted into the uterus of a host mother who has been hormonally prepared to accept the embryo. The baby sheep who is born is genetically identical to the sheep who provided the donor nucleus (except for small amounts of nonnuclear DNA from the donor egg cell).
The entire process, called nuclear transfer, is complicated and requires very careful coordination and timing. The manipulation of individual cells and nuclei is technically demanding, and there are many more failures than successes.
But at What Cost?
Even if nuclear transfer works and a cloned embryo is successfully implanted into a host mother, problems often develop. There is a higher rate of miscarriage among cloned embryo pregnancies, and cloned animals that are born are often larger than normal and have reduced survival rates. It was recently announced that Dolly the sheep, the first mammalian clone, has developed arthritis at the age of five. While it is impossible to tell if the arthritis is the result of cloning, it does raise a flag about the procedures and their implications for animal welfare.
The next five to 10 years are likely to provide additional insights into the world of cloning. While promises are being made about the tremendous benefits this research will provide for both human and animal health, great care must be taken to keep the costs in sight. If the potato clone doesn’t work out, the potato doesn’t suffer. At this point, we can’t say the same thing for animals.
Dr. “Z” is senior vice president of ASPCA Animal Sciences and Animal Watch science advisor.
Courtesy of
ASPCA
424 East 92nd St.
New York, NY 10128-6804
(212) 876-7700
www.aspca.org
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