25 years since Dolly, the world's first cloned sheep, was born
Author: Bisong Ko
Eitor: Jiho Jang
From an artificial embryo twinning demonstration with sea urchin for the first time in 1885 to somatic cell nuclear transformation to creating a human embryo that could be used as a source of embryonic stem cells, cloning has come a long way since Dolly the sheep was the first mammal to have been successfully cloned from an adult cell. Cloning is a complex process that lets one exactly copy the genetic, or inherited, traits of an animal. Livestock species that scientists have successfully cloned are cattle, swine, sheep, and goats. Scientists have also cloned mice, rats, rabbits, cats, mules, horses and one dog.
Dolly was cloned from a mammary gland cell taken from an adult Finn Dorset ewe. British developmental biologist Ian Wilmut and colleagues of the Roslin Institute created her by using electrical pulses to fuse the mammary cell with an unfertilized egg cell, the nucleus of which had been removed. The fusion process resulted in the transfer of the mammary cell nucleus into the egg cell, which then began to divide. In order for the mammary cell nucleus to be accepted and functional within the host egg, the cell first had to be induced to abandon the normal cycle of growth and division and enter a quiescent stage. To accomplish that, researchers deliberately withheld nutrients from the cells. The importance of the step had been determined experimentally, though an explanation for its necessity was lacking. Nevertheless, starting with a collection of mammary cell nuclei and host egg cytoplasms derived from Scottish Blackface ewes, a number of fused couplets successfully formed embryos. The reconstructed embryos were transferred to surrogate Scottish Blackface ewes. Of 13 recipient ewes, one became pregnant, and 148 days later, which is essentially normal gestation for a sheep, Dolly was born.
Dolly remained alive and well long after her birth, with a functional heart, liver, brain, and other organs, all derived genetically from the nuclear DNA of an adult mammary gland cell. The technique used to produce her later became known as somatic cell nuclear transfer (SCNT). SCNT has since been used to generate a wide variety of mammalian clones, from different types of adult cells; its success in producing clones of primates, however, has been notably limited.
Nevertheless, researchers keep studying about the technique of cloning. It is due to the fact as follows: (1) Cloning animal models of disease (2) Cloning to make stem cells (3) Reviving extinct speicies (4) Drug production (5) Cloning humans. First, many researchers learn about human disease via the study of animal models such as mice. It is because animal models are often genetically engineered to carry disease-causing mutations in their genes. The creation of transgenic animals requires an intensive process with trial-and-error and several generations of breeding. Cloning can reduce the time needed to make a transgenic animal model, resulting a population of genetically identical animals of study. Second, stem cells build, maintain, and repair our bodies throughout our lives. Since these are natural processes of stem cells, they can be manipulated to treat damaged or diseased organs and tissues. These cells can be used for medical purposes, even to grow entire organs. In third, in 2009, scientists had their first near-success resurrecting an extinct animal. Using goats as egg donors and surrogates, they made several clones of a wild mountain called the bucardo surviving clone who died soon after birth. Only frozen tissue samples come from a female, so there is a limitation of only producing female clones. However, scientists speculate they may be able to remove one X chromosome and add a Y chromosome from a related goat species to make a male. In fourth, farm animals such as cows, sheep, and goats are genetically engineered to produce drugs or proteins that are useful for medicine. For example, scientists could collect cells from cows that produce large amounts of milk, and then they could insert genes that encode drugs or vaccines into the cell's DNA. If they take a nucleus from one of these cells and transfer it to a cow egg, it can develop into the cow that makes the drug. Since every cell in a cow has a drug-gene, it can pass that gene on to its offspring and create a whole herd of cows that produce drugs. What's more, we can avoid the problem of genetic reshuffling that occurs during sexual reproduction and simply clone cows that produce drugs. Lastly, the purpose is to clone human in some day. The prospect of human cloning is highly controversial, and it raises many ethical, legal and social issues to be considered. Most people think that human reproductive cloning, which is a clone to make a human baby, is immoral. On the other hand, supporters of human cloning see it as a possible solution to the fertility problem. It also imagines creating replicas of geniuses who can develop society. Although many species have been successfully cloned, the process is still technically difficult and inefficient. The success rate of cloning is very low: most embryos do not develop, and many pregnancies end in miscarriage. Current efforts in human cloning focus on creating embryonic stem cells for research and medicine. However, many people think that this kind of therapeutic cloning is dangerously close to human reproductive cloning.
In addition to the ethical issues in livestock cloning, although cloning may eventually become an important technology for livestock production, there are four ethical issues that must be addressed before the practice becomes widespread. Researchers must prove that the procedure is not harmful to the health or well-being of the affected animals. Also, animal research institutes should evaluate the social net benefit of livestock farmers by comparing the opportunity cost of research capabilities lost in biomedical projects. Scientists should consider the indirect impact of cloning research on greater ethical issues surrounding human cloning. Moreover, the market structure of cloned animal products must protect individual choices and recognize that many individuals abhor the prospect of cloning (Thompson).
Nuclear cloning involves the production of genetically identical animals to donor cells used in a technique known as nuclear transfer (NT). However, now it is an inefficient process: in cattle, only about 6% of embryos transferred to the reproductive organs of the receiving cows produce healthy, long term surviving clones. Reproductive cloning is a very inefficient technique and most cloned animal embryos cannot develop into healthy individuals. For instance, Dolly was the only clone to be born live out of a total of 277 cloned embryos. This very low efficiency, combined with safety concerns, presents a serious obstacle to the application of reproductive cloning. Furthermore, researchers observed adverse health effects in cloned sheep and other mammals. This includes an increase in birth size and various defects in important organs such as the liver, brain, and heart. Other consequences include premature aging and immune system problems. Another potential problem is the relative age of chromosomes in cloned cells. As the cells undergo a normal process of division, the ends of chromosomes called telomeres contract. Over time, telomeres become too short for cells to divide further, resulting in cell death. As a result, clones made from cells collected from adults may already have chromosomes shorter than normal, which can shorten the cell life of the clones. Indeed, Dolly, who was cloned from the cell of a 6-year-old sheep, had chromosomes that were shorter than those of other sheep her age. Dolly died when she was six years old, about half the average sheep's 12-year lifespan (Wells DN).
To conclude, on one hand, cloning can be seen as a revolution of the technology. It is because the technique of cloning can be used to clone animal models of disease, to clone to make stem cells, to revive extinct speicies, to produce drugs, and finally to clone humans. However, on the other hand, the author think that the technique of NT is inefficient. It is because the donor nucleus has a great possibility of failing to express early embryonic genes and establish a normal embryonic pattern of chromatin modifications. In addition to the aspect of unethical reasoning, the technique undermines the dignity of the human person. Using human embryos as experimental tools is an unethical act that destroys the dignity of human life, just as using grown humans as experimental tools, because human embryos are already full life with life and are becoming full human beings.
Reference:
[List of all external resources utilized, APA-cited and enumerate alphabetically]
What is cloning?
https://www.fda.gov/animal-veterinary/animal-cloning/primer-cloning-and-its-use-livestock-operations
History of Animal Cloning:
https://learn.genetics.utah.edu/content/cloning/clonezone https://www.bbc.com/future/article/20120229-cloning-which-animals-and-when
Dolly the sheep
Purpose of cloning
Problems and prospects
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