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. But as shown by Cc the cloned cat, a clone may not turn out exactly like the original pet whose DNA was used to make the clone.
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 cloned embryos.
This very low efficiency, combined with safety concerns, presents a serious obstacle to the application of reproductive cloning. Researchers have observed some adverse health effects in sheep and other mammals that have been cloned. These include an increase in birth size and a variety of defects in vital organs, such as the liver, brain and heart. Other consequences include premature aging and problems with the immune system.
Another potential problem centers on the relative age of the cloned cell's chromosomes. As cells go through their normal rounds of division, the tips of the chromosomes, called telomeres, shrink. Over time, the telomeres become so short that the cell can no longer divide and, consequently, the cell dies.
This is part of the natural aging process that seems to happen in all cell types. As a consequence, clones created from a cell taken from an adult might have chromosomes that are already shorter than normal, which may condemn the clones' cells to a shorter life span. 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 year lifespan. Therapeutic cloning involves creating a cloned embryo for the sole purpose of producing embryonic stem cells with the same DNA as the donor cell. These stem cells can be used in experiments aimed at understanding disease and developing new treatments for disease. To date, there is no evidence that human embryos have been produced for therapeutic cloning. The richest source of embryonic stem cells is tissue formed during the first five days after the egg has started to divide.
At this stage of development, called the blastocyst, the embryo consists of a cluster of about cells that can become any cell type. Stem cells are harvested from cloned embryos at this stage of development, resulting in destruction of the embryo while it is still in the test tube. Researchers hope to use embryonic stem cells, which have the unique ability to generate virtually all types of cells in an organism, to grow healthy tissues in the laboratory that can be used replace injured or diseased tissues.
In addition, it may be possible to learn more about the molecular causes of disease by studying embryonic stem cell lines from cloned embryos derived from the cells of animals or humans with different diseases. Finally, differentiated tissues derived from ES cells are excellent tools to test new therapeutic drugs.
Many researchers think it is worthwhile to explore the use of embryonic stem cells as a path for treating human diseases. However, some experts are concerned about the striking similarities between stem cells and cancer cells. Both cell types have the ability to proliferate indefinitely and some studies show that after 60 cycles of cell division, stem cells can accumulate mutations that could lead to cancer.
Therefore, the relationship between stem cells and cancer cells needs to be more clearly understood if stem cells are to be used to treat human disease. Gene cloning is a carefully regulated technique that is largely accepted today and used routinely in many labs worldwide.
However, both reproductive and therapeutic cloning raise important ethical issues, especially as related to the potential use of these techniques in humans. Reproductive cloning would present the potential of creating a human that is genetically identical to another person who has previously existed or who still exists. This may conflict with long-standing religious and societal values about human dignity, possibly infringing upon principles of individual freedom, identity and autonomy.
However, some argue that reproductive cloning could help sterile couples fulfill their dream of parenthood. Others see human cloning as a way to avoid passing on a deleterious gene that runs in the family without having to undergo embryo screening or embryo selection.
Therapeutic cloning, while offering the potential for treating humans suffering from disease or injury, would require the destruction of human embryos in the test tube. Consequently, opponents argue that using this technique to collect embryonic stem cells is wrong, regardless of whether such cells are used to benefit sick or injured people.
Cloning Fact Sheet. Do clones ever occur naturally? What are the types of artificial cloning? How are genes cloned? How are animals cloned? The primary approach to screening nucleic acid sequences for vector contamination is to run a sequence similarity search against a database of vector sequences. VecScreen then categorizes the matches, eliminates redundant hits, and shows the location of contaminating and suspect segments on a simple graphical display.
Another method used to detect vector contamination is to search the sequence for restriction sites. Software for restriction site analysis is widely available. Sequences can also be analyzed via the Internet using Webcutter. Clusters of restriction sites often indicate sequence derived from the multiple cloning site MCS of a vector. The restriction site analysis method has the advantage of being able to reveal contamination with a MCS, even if the vector sequence is not in any database. The ability to detect vector contamination using this approach alone is limited, however, because it is hard to distinguish between a single cloning site and a naturally occurring restriction site, and because the cloning process does not always recreate the sites used for cloning.
Restriction site analysis is therefore best used to supplement a sequence similarity based search, such as VecScreen. VecScreen can be used to detect contamination with many of the adapters, linkers, and PCR primers used in the most popular cDNA cloning strategies because the UniVec database includes the sequences for such oligonucleotides.
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