Researchers routinely use cloning techniques to make copies of genes that they wish to study. The procedure consists of inserting a gene from one organism, often referred to as "foreign DNA," into the genetic material of a carrier called a vector.
Examples of vectors include bacteria, yeast cells, viruses or plasmids, which are small DNA circles carried by bacteria. After the gene is inserted, the vector is placed in laboratory conditions that prompt it to multiply, resulting in the gene being copied many times over.
In reproductive cloning, researchers remove a mature somatic cell , such as a skin cell, from an animal that they wish to copy.
They then transfer the DNA of the donor animal's somatic cell into an egg cell, or oocyte, that has had its own DNA-containing nucleus removed. Researchers can add the DNA from the somatic cell to the empty egg in two different ways.
In the first method, they remove the DNA-containing nucleus of the somatic cell with a needle and inject it into the empty egg.
In the second approach, they use an electrical current to fuse the entire somatic cell with the empty egg. In both processes, the egg is allowed to develop into an early-stage embryo in the test-tube and then is implanted into the womb of an adult female animal. Ultimately, the adult female gives birth to an animal that has the same genetic make up as the animal that donated the somatic cell.
This young animal is referred to as a clone. Reproductive cloning may require the use of a surrogate mother to allow development of the cloned embryo, as was the case for the most famous cloned organism, Dolly the sheep. Over the last 50 years, scientists have conducted cloning experiments in a wide range of animals using a variety of techniques.
In , researchers produced the first genetically identical mice by splitting mouse embryos in the test tube and then implanting the resulting embryos into the wombs of adult female mice. Shortly after that, researchers produced the first genetically identical cows, sheep and chickens by transferring the nucleus of a cell taken from an early embryo into an egg that had been emptied of its nucleus. It was not until , however, that researchers succeeded in cloning the first mammal from a mature somatic cell taken from an adult animal.
After attempts, Scottish researchers finally produced Dolly, the lamb from the udder cell of a 6-year-old sheep. Two years later, researchers in Japan cloned eight calves from a single cow, but only four survived. Besides cattle and sheep, other mammals that have been cloned from somatic cells include: cat, deer, dog, horse, mule, ox, rabbit and rat. In addition, a rhesus monkey has been cloned by embryo splitting.
Despite several highly publicized claims, human cloning still appears to be fiction. There currently is no solid scientific evidence that anyone has cloned human embryos. In , scientists in South Korea claimed to have successfully cloned a human embryo, but said the experiment was interrupted very early when the clone was just a group of four cells.
In , Clonaid, part of a religious group that believes humans were created by extraterrestrials, held a news conference to announce the birth of what it claimed to be the first cloned human, a girl named Eve.
However, despite repeated requests by the research community and the news media, Clonaid never provided any evidence to confirm the existence of this clone or the other 12 human clones it purportedly created.
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.
The answer lies in the X chromosome. In cats, a gene that helps determine coat color resides on this chromosome. Both CC and Rainbow, being females, have two X chromosomes. Males have one X and one Y chromosome. Since the two cats have the exact same X chromosomes, they have the same two coat color genes, one specifying black and the other specifying orange.
Very early in her development, each of Rainbow's cells "turned off" oneentire X chromosome, thereby turning off either the black or the orange color gene. This process, called X-inactivation, happens normally in females, in order to prevent them from having twice as much X-chromosome activity as males.
It also happens randomly, meaning that different cells turn off different X chromosomes. So like all female mammals, Rainbow developed as a mosaic.
Each cell that underwent X-inactivation gave rise to a patch of cells that had oneor the other coat color gene inactivated. Some patches specified black,other patches specified orange, and still others specified white, due to more complex genetic events. This is how all calico cats, like Rainbow, get their markings. CC looks different because she was made from a somatic cell from Rainbow in which the X-chromosome with the orange gene had been inactivated; only the black gene was active.
What's interesting is that, as CC developed, her cells did not change the inactivation pattern. Therefore, unlike Rainbow, CC developed without any cells that specified orange coat color. The result is CC's black and white tiger-tabby coat. Rainbow and CC are living proof that a clone will not look exactly like the donor of its genetic material. Programs are underway to clone agricultural animals, such as cattle and pigs, that are efficient producers of high-quality milk or meat.
A group of researchers at Utah State University led by Dr. Their aim isn't to produce animals for consumption—cloning is far more labor-intensive and expensive than conventional breeding methods. Instead, they want to use these animals as breeding stock. The important thing to know about beef cattle is that the quality and yield of their meat can be assessed only after they are slaughtered.
And male animals are routinely neutered when they're a few days old. That is, their testes are removed, so they are unable to make sperm. But cells from a high-quality carcass can be cloned, giving rise to an animal that is able, though conventional breeding methods, to pass its superior genes to its offspring.
Scientists have also cloned mules, a reproductively sterile hybrid of a male donkey and a female horse; dairy cows; and horses. One gelded racing horse, a male whose testes have been removed, has a clone that is available for breeding. Some of the cloned cows produce about twice as much milk as the average producer. And a cloned racing mule is ranked among the best in the world.
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. They still have not cloned a human, though.
In part, this is because it is difficult to produce a viable clone. In each attempt, there can be genetic mistakes that prevent the clone from surviving. It took scientists attempts to get Dolly right. There are also ethical concerns about cloning a human being. Researchers can use clones in many ways. An embryo made by cloning can be turned into a stem cell factory. Stem cells are an early form of cells that can grow into many different types of cells and tissues.
Scientists can turn them into nerve cells to fix a damaged spinal cord or insulin-making cells to treat diabetes. Support us Find out more. Receive our newsletter. This website uses cookies to improve your experience. We'll assume you're ok with this, but you can opt-out if you wish. Accept Read More. Cookies Policy. Close Privacy Overview This website uses cookies to improve your experience while you navigate through the website.
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