Cloning

Date: 2015-10-07; view: 587.

Cloning, creating a copy of living matter, such as a cell or organism. The copies produced through cloning have identical genetic makeup and are known as clones. Many organisms in nature reproduce by cloning. Scientists use cloning techniques in the laboratory to create copies of cells or organisms with valuable traits. Their work aims to find practical applications for cloning that will produce advances in medicine, biological research, and industry.

I OVERVIEW

Genetic Engineering enables scientists to produce clones of cells or organisms that contain the same genes. Scientists are getting better at all kinds of cloning – from individual cells to entire organisms. The results are as follows: 1) Certain primitive cells found in the brain, blood and elsewhere remain undeveloped enough even in adults that they can grow into a limited number of cell types. The cells might be coaxed to become a wider variety of tissues. 2) Researchers have successfully isolated stem cells from 5-day-old human embryos that were created during in-vitro fertilization and would otherwise have been destroyed. Unlike adult stem cells, embryonic stem cells can develop into any of more than 200 tissues in the body, fron insulin-producing cells that may one day treat diabetes to heart cells that may repair cardiac muscle. 3) Since adult stem cells are rare, scientists may be able to use tissue that is more easily available, such as the skin. In one method, researchers replace the genetic material from a donor egg with the nucleus of a skin cell; the new hybrid begins to grow like an embryo from which stem cells can be isolated. These in turn could produce transplants that are immunologically identical to the original host and therefore would not be rejected.

Farmers started cloning plants thousands of years ago in simple ways, such as taking a cutting of a plant and letting it root to make another plant. Early farmers also devised breeding techniques to reproduce plants with such characteristics as faster growth, larger seeds, or sweeter fruits. They combined these breeding techniques with cloning to produce many plants with desired traits. These early forms of cloning and breeding were slow and sometimes unpredictable. By the late 20th century scientists developed genetic engineering, in which they manipulate deoxyribonucleic acid (DNA), the genetic material of living things, to more precisely modify a plant's genes. Scientists combine genetic engineering with cloning to quickly and inexpensively produce thousands of plants with a desired characteristic.

Cloning techniques can also be applied to animals. Scientists generate genetically modified animals with new traits, such as the ability to resist disease, and they use cloning techniques to reproduce these genetically modified animals. In the near future scientists hope to bolster populations of endangered species by cloning members from existing populations. Someday scientists may even resurrect extinct species by cloning cells from preserved specimens.

Industry also utilizes cloning technology. For example, some bacteria eat toxic substances, such as gasoline or industrial chemicals, that are common pollutants. These bacteria can be cloned to make legions of bacteria with the ability to clean up environmental contamination (see Bioremediation). Likewise, cloned animals can be used to make a variety of ingredients, such as proteins, that are used in many commercial products.

Perhaps most important from a human perspective, cloning promises great advances in medicine. Scientists have already inserted fragments of DNA containing the human gene for a blood-clotting protein into cells of a sheep. Through cloning techniques, scientists have generated new sheep whose milk contains the protein, which is needed by people with the blood-clotting disorder known as hemophilia. In the near future, researchers hope to use cloning to develop animals with human diseases and use these cloned animals to test the safety and effectiveness of new treatments devised for humans. Biomedical scientists hope to take cells from an ill patient, genetically modify them, and clone the modified cells to grow exactly the cells that the patient needs to regain health. Some scientists even imagine a day when cloning could be part of a process that grows entire organs for transplants.

II HOW SCIENTISTS CLONE CELLS

Scientists initially made cloned cells in the laboratory by letting a single cell divide into a population of genetically identical cells. In this process scientists put the original cell in a laboratory dish containing culture medium (nutrients needed to keep a cell alive). The cell's natural process of mitosis (cell division) then produces genetically identical offspring. This process mimics how cells multiply, for instance, in plants and in the human body.

Scientists later developed more complex cloning techniques using animal embryos. Every cell in an animal arises from a fertilized egg. The fertilized egg divides to form an embryo, and each cell in the embryo has the same genetic makeup. At some point in the embryo's growth and development, cells differentiate and become specialized. For instance, a heart cell only functions in the heart and not the liver, even though the genes of a heart cell and liver cell are the same.

In the 1950s scientists began to experiment with embryo cells that were undifferentiated—that is, they had not yet specialized into a particular type of cell. Scientists found that such embryo cells are totipotent (able to give rise to all the different cell types in the body). Exploiting this characteristic, scientists developed three techniques to clone embryo cells: blastomere separation, blastocyst division, and somatic cell nuclear transfer.

Medical procedures using stem cells still remain experimental. In 2001 the first clinical trial that injected stem cells into the brains of patients suffering from Parkinson disease produced mixed results. Although the injected cells grew, the treatment produced no obvious benefits for patients aged 60 and older. Some of the patients under age 60 said they felt better after the treatment, but about 15 percent of these younger patients acquired irreversible side effects, including twitching and other uncontrollable movements.

Cloned stem cells could pose other risks. For example, the cloning process—producing large numbers of cells from one starting cell—could create genetic errors in the cells. If something went wrong in cell division during cloning, the error could be replicated in many other cells—even all of them if the error existed in the original cell. Nevertheless, in 2002 scientists at Rutgers University found few genetic mutations in embryonic stem cells cloned from mice. In fact, the study's investigators found those stem cells were better able to resist mutation than some adult cells.

Some scientists worry that cloned stem cells could carry disease. For example, when cloning stem cells, scientists typically mix human stem cells with mouse cells in culture. The mouse cells produce an as yet unidentified nutrient or growth factor that helps keep the human stem cells alive. Scientists worry that infected mouse cells could just as easily transfer viruses to the human stem cells. They hope to develop new methods of cell culture that do not rely on such “feeder cells.”

III HISTORY OF CLONING

Laboratory cloning techniques using undifferentiated embryo cells were first developed in the late 1800s, when German zoologist Hans Dreisch separated a sea urchin embryo when it was just two cells, and both cells grew to adults. In the early 1900s, German embryologist Hans Spemann extended Dreisch's work to salamanders. In his experiments Spemann determined that a nucleus from a salamander embryo cell could direct the development of a complete organism. He published his results in 1938 and proposed a “fantastical” experiment to produce an animal by removing the nucleus from one cell and placing it into an egg cell with its nucleus removed.

In 1952 Spemann's proposed experiment became reality when American biologists Robert Briggs and Thomas King used cell nuclear transfer to insert DNA from a frog embryo cell into an enucleated frog egg. The resulting embryo grew into an adult. These early cloning experiments using cell nuclear transfer were successful only when the donor DNA was taken from an embryonic cell.

In 1962 British developmental biologist John Gurdon began cloning experiments using nonembryonic cells—specifically, cells from the intestinal lining of tadpoles. Gurdon believed that the tadpoles were old enough so that cells taken from them would be differentiated. Gurdon exposed a frog egg to ultraviolet light, which destroyed its nucleus. He then removed the nucleus from the tadpole intestinal cell and implanted it in the enucleated egg. The egg grew into a tadpole that was genetically identical to the DNA-donating tadpole.

But the tadpoles cloned in Gurdon's experiments never survived to adulthood and scientists now believe that many of the cells used in these experiments may not have been differentiated cells after all. Nevertheless, Gurdon's experiments captured the attention of the scientific community and the tools and techniques he developed for nuclear transfer are still used today. The term clone (from the Greek word klōn, meaning “twig”) had already been in use since the beginning of the 20th century in reference to plants. In 1963 the British biologist J. B. S. Haldane, in describing Gurdon's results, became one of the first to use the word clone in reference to animals.

Scientists soon turned their attention to cloning mammals, which proved even more complex than earlier cloning experiments on invertebrates and amphibians. In 1977 German developmental biologist Karl Illmensee reported cloning mice from cells derived from early embryos. But Illmensee's findings were largely discredited because he used questionable laboratory techniques. Many agricultural researchers tried to clone cattle using somatic cell nuclear transfer, but it was not until 1984 that Danish biologist Steen Willadsen, working at Cambridge University in England, created the first cloned mammal. Willadsen cloned sheep by using nuclear transfer with DNA from early embryonic cells. Two years later, a team of researchers at the University of Wisconsin cloned a cow through a similar approach.

In the 1990s cloning techniques advanced rapidly. In 1995 British scientists Keith Campbell and Ian Wilmut at the Roslin Institute cloned two lambs, named Megan and Morag, from embryonic cells. In this experiment, the scientists were able to keep the embryonic cells alive in culture for some time before beginning the cloning procedure. This advance enabled scientists to modify an embryonic cell's genes in culture before cloning it to produce genetically modified livestock.

Dolly the Cloned Sheep In 1996 a sheep named Dolly was successfully cloned from a cell of an adult female sheep. This advance proved that adult cells could provide the cloning potential of embryonic cells, enabling scientists to choose the mature individual they want to duplicate. Using cells from immature animals makes it more difficult for scientists to predict with certainty the physical characteristics of the resultant clone.

Scientists then began to focus their efforts on cloning a mammal with donor DNA from an adult cell. Scientists at the Roslin Institute succeeded in 1996 when the cloned sheep Dolly was born. Dolly came from a cell taken from an udder of an adult Finn Dorsett sheep and an enucleated egg from a Scottish blackface ewe. Dolly's birth proved that adult cells could acquire the cloning potential of embryonic cells. Like other efforts in cloning, however, this work demanded perseverance—it took 277 tries at somatic cell nuclear transfer to create Dolly.

First Cloned Cat The world's first cat clone, named "CC," for carbon copy or courtesy copy, was produced by scientists at Texas A&M University in College Station. Born December 22, 2001, the kitten was cloned using a method called nuclear transfer, in which nuclei from cells of an adult animal are inserted into egg cells with nuclei removed. The embryos that result are then implanted into the uterus of a surrogate mother, where they develop in a normal pregnancy.

Since the cloning of Dolly the sheep in 1996, scientists have cloned a wide variety of mammals from adult cells, including cows, goats, pigs, cats, and rabbits. While scientists have achieved some remarkable advances in animal cloning, drawbacks remain. Somatic cell nuclear transfer is inefficient—few cloned embryos survive through birth. For example, in experiments to create the first cloned rabbits in 2001, scientists implanted 371 embryos into surrogate mothers, but only six cloned rabbits were born.

Despite these drawbacks, scientists believe that animal cloning will one day advance agricultural practices and medicine, and even prevent the extinction of endangered animals. In agriculture, cloned cattle could produce a higher yield of meat or milk. The pharmaceutical industry already uses cloned animals to produce drugs for human use. For example, PPL Therapeutics in Scotland has generated sheep that produce milk containing a protein that helps in the treatment of hemophilia. One day pharmaceutical firms may clone large populations of genetically modified animals to quickly and inexpensively derive this protein for use in drug products.

Cloned animals could also improve laboratory experiments. Researchers could create many genetically identical animals to reduce the variability in a sample population used in experiments, making it easier for scientists to evaluate disease. Moreover, scientists could clone a large number of animals that suffer from a human disease, such as arthritis, to study the disease's progression and potential treatments. Some cloned animals such as sheep and pigs live for years, and scientists could use these animals to evaluate their long-term response to drug treatments.

IV Can Humans Be Cloned?

If scientists can clone animals, can they clone humans? In 1998 a Korean research team announced that it cloned a human embryo through somatic cell nuclear transfer, but the embryo only survived to four cells. In 2001 researchers at the biotechnology firm Advanced Cell Technology claimed to clone human embryos that divided to six cells before dying. Many scientists argue that because the embryos from these two experiments did not double their cell size every 24 hours, they could not be considered true human embryos. In any case, scientists feel it is only a matter of time before scientists resolve technical obstacles to human cloning.

Beyond safety, the possibility of cloning humans also raises a variety of social issues. What psychological issues would result for a cloned child who is the identical twin of his or her parent? How will a cloned child deal with the pressures of being compared to its genetic donor? A clone will never be identical to the genetic donor because environmental differences will influence the clone's development. Still, a cloned boy created from basketball star Michael Jordan's genetic material, for example, could suffer considerable criticism if he decided to pursue classical piano instead of slam-dunking.

Are these issues compelling enough to ban the cloning of humans? Although some scholars argue that a clone might face unique problems, most offspring face some sort of burden. Children from poor families, for example, suffer some hardships that children from wealthy homes never imagine. Children in some developing nations face a tougher life than children in the United States. Nevertheless, few people would encourage a ban against having babies because of financial status or where a person lives. Cloning proponents argue that human cloning should not be banned simply because of potential hardships for the offspring.

If human cloning ever becomes an option for parents, financial status could play a role because cloning would probably be expensive and only available to the wealthy. Accordingly, wealthy families might use cloning to give their offspring the best characteristics imaginable. Scientists could use genetic engineering to put together genes for such characteristics as beauty or intelligence, and then clone the cell to make a super child of sorts. If that capability was only available to wealthy people, the divide between the wealthy and the poor could widen farther than ever imagined.

Soon after the cloning of the first human embryos in 2001, the Roman Catholic Church condemned such research. Many other religions agree that human cloning should be entirely and forever banned. Theologians view cloning as a thorny issue, an example of the ongoing tension between faith and science. Some people believe the scientific advances that enable human cloning are a God-given blessing. Others argue that scientists should not presume to play God by manipulating human genetic makeup. Some opponents claim that cloning must be forbidden because it involves destroying human embryos—such as the ones used to harvest stem cells. These opponents argue that any embryo is a viable human being and should never be destroyed intentionally.

The apparently successful cloning of an adult mammal, announced in 1997 by a team of Scottish scientists led by Ian Wilmut, raised a number of ethical issues. The cloned mammal, a sheep named Dolly, seemed to open the theoretical possibility that human beings could also be cloned. Immediately after Dolly's existence was announced, politicians in several countries called for a ban on scientific research into human cloning. In this article British philosopher Mary Warnock warns against letting the fear of cloning cripple the future of scientific research into its possibilities.

Answer the following questions

1. What is cloning?
2. What are the main landmarks in the history of cloning?
3. What are the latest achievements of scientists in the field of embryology?
4. What are animals, plants and cells cloned for?

Most of the concerns center on efforts to create clones of human beings. Some people might want to make a human clone because they would have a child with certain characteristics. Most scientists seem interested in cloning in order to learn what they can about how genes affect the development of an organism from the embryo to adulthood. Scientists and common people are concerned about the ethical considerations that need to be addressed in cloning humans and animals, whether human cloning should be banned. Read the following extract from a newspaper article and express your point of view on the problem.

After Dolly: The Future of Cloning