In , a group led by Woo-Suk Hwang of Seoul National University in South Korea published a paper in the journal Science in which it claimed to have created a cloned human embryo in a test tube. However, an independent scientific committee later found no proof to support the claim and, in January , Science announced that Hwang's paper had been retracted. From a technical perspective, cloning humans and other primates is more difficult than in other mammals.
One reason is that two proteins essential to cell division, known as spindle proteins, are located very close to the chromosomes in primate eggs. Consequently, removal of the egg's nucleus to make room for the donor nucleus also removes the spindle proteins, interfering with cell division. In other mammals, such as cats, rabbits and mice, the two spindle proteins are spread throughout the egg. So, removal of the egg's nucleus does not result in loss of spindle proteins.
In addition, some dyes and the ultraviolet light used to remove the egg's nucleus can damage the primate cell and prevent it from growing. Clones do not always look identical. Although clones share the same genetic material, the environment also plays a big role in how an organism turns out. For example, the first cat to be cloned, named Cc, is a female calico cat that looks very different from her mother. The explanation for the difference is that the color and pattern of the coats of cats cannot be attributed exclusively to genes.
A biological phenomenon involving inactivation of the X chromosome See sex chromosome in every cell of the female cat which has two X chromosomes determines which coat color genes are switched off and which are switched on.
The distribution of X inactivation, which seems to occur randomly, determines the appearance of the cat's coat. Reproductive cloning may enable researchers to make copies of animals with the potential benefits for the fields of medicine and agriculture. For instance, the same Scottish researchers who cloned Dolly have cloned other sheep that have been genetically modified to produce milk that contains a human protein essential for blood clotting.
The hope is that someday this protein can be purified from the milk and given to humans whose blood does not clot properly. Another possible use of cloned animals is for testing new drugs and treatment strategies.
The great advantage of using cloned animals for drug testing is that they are all genetically identical, which means their responses to the drugs should be uniform rather than variable as seen in animals with different genetic make-ups.
After consulting with many independent scientists and experts in cloning, the U. Food and Drug Administration FDA decided in January that meat and milk from cloned animals, such as cattle, pigs and goats, are as safe as those from non-cloned animals. The FDA action means that researchers are now free to using cloning methods to make copies of animals with desirable agricultural traits, such as high milk production or lean meat.
However, because cloning is still very expensive, it will likely take many years until food products from cloned animals actually appear in supermarkets.
Another application is to create clones to build populations of endangered, or possibly even extinct, species of animals. In , researchers produced the first clone of an endangered species: a type of Asian ox known as a guar.
Sadly, the baby guar, which had developed inside a surrogate cow mother, died just a few days after its birth. In , another endangered type of ox, called the Banteg, was successfully cloned.
Soon after, three African wildcats were cloned using frozen embryos as a source of DNA. Although some experts think cloning can save many species that would otherwise disappear, others argue that cloning produces a population of genetically identical individuals that lack the genetic variability necessary for species survival.
Some people also have expressed interest in having their deceased pets cloned in the hope of getting a similar animal to replace the dead one.
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. Chickens and other poultry have not been cloned. Most people think of livestock breeding taking place through traditional mating, in which males and females physically get together to reproduce.
In fact, this is not often the case. Traditional mating is not that efficient, if the goal is to produce as many offspring as possible. For example, a male has enough sperm to produce many more offspring than would be possible by traditional mating. Traditional mating also has certain risks: one or both of the animals may be injured in the process of mating.
The female may be hurt by the male because he is often much larger, or an unwilling female may injure the male. There is also a risk of infection or transmission of venereal disease during traditional mating. Because of these factors, many farmers use assisted reproductive technologies ARTs for breeding. These include artificial insemination, embryo transfer, and in vitro fertilization a process by which egg and sperm are united outside the body.
Artificial insemination was first documented in the breeding of horses in the 14th century. The first successful embryo transfer of a cow was in , and the first in vitro fertilization IVF -derived animal was a rabbit born in Livestock production in the United States now uses all these methods regularly. The frozen semen can come from a bull many miles, or even many states, away.
Cloning is a more advanced form of these assisted reproductive technologies. Much of the public perception of cloning likely comes from science fiction books and movies. Some people incorrectly believe that clones spring forth fully formed, or are grown in test tubes. This is just not the case. Clones are born just like other animals. They are similar to identical twins, only born at different times. Just as twins share the same DNA, clones have the same genes as the donor animal.
A clone is not a mutant, nor is it a weaker version of the original animal. In all of the other assisted reproductive technologies, the male and female parents each contribute half of their genes to their offspring.
Farmers have worked for years to choose animals with the best traits and breed them together. This increases the chance these good traits will be passed on and become more common in livestock herds. Thus, a farmer who clones an especially desirable but aging or injured animal knows in advance that the clone will have the genetic potential to be an especially good, younger animal.
He can then use that animal to further reproduce by traditional mating or other ARTs. Most cloning today uses a process called somatic cell nuclear transfer SCNT. Just as with in vitro fertilization, scientists take an immature egg, or oocytes, from a female animal often from ovaries obtained at the slaughterhouse.
This leaves behind the other components necessary for the initial stages of embryo development. Scientists then add the nucleus or cell from the donor animal that has the desirable traits the farmer wishes to copy.
In order to practice the techniques, scientists have so far cloned non-endangered animals, namely, tadpoles, mice and even horses. It took thousands of attempts for these experiments to be a success. Dolly was a clone produced by reproductive cloning.
She was produced by reproductive cloning but died aged six half the normal age for a sheep due to arthritis and a lung tumour. Scientists clone cells in the laboratory using SCNT so that they can understand disease and test medicines. This is known as therapeutic cloning.
One day, scientists hope that cloned cells will be used to treat serious diseases such as heart problems, diabetes and spinal injuries. Scientists hope to remove stem cells from a five day old cloned embryo and grow specific cell lines from them which may treat disease. Single-celled organisms like bacteria make exact copies of themselves each time they reproduce. In humans, identical twins are similar to clones.
They share almost the exact same genes. Identical twins are created when a fertilized egg splits in two. Scientists also make clones in the lab. They often clone genes in order to study and better understand them. To clone a gene , researchers take DNA from a living creature and insert it into a carrier like bacteria or yeast. Every time that carrier reproduces, a new copy of the gene is made. Animals are cloned in one of two ways. The first is called embryo twinning.
Scientists first split an embryo in half. Each part of the embryo develops into a unique animal, and the two animals share the same genes. The second method is called somatic cell nuclear transfer. Somatic cells are all the cells that make up an organism, but that are not sperm or egg cells. Somatic cells, on the other hand, already contain two full sets of chromosomes.
The egg develops into an embryo that contains the same genes as the cell donor. In , Scottish scientists cloned the first animal, a sheep they named Dolly. She was cloned using an udder cell taken from an adult sheep.
Since then, scientists have cloned cows, cats, deer, horses, and rabbits.
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