Cloning

A clone is a group of genetically identical cells descended from a single common ancestor. A clone can describe a group of cells or a multicellular organism. In both cases, the clone or offspring has the exact same genes as the parent organism.

A clone or a genetic double is not as rare in the natural world as one might suppose. Besides identical twins (who are the result of a fertilized egg separating completely during its two-cell stage), there are numerous examples in the plant kingdom. Almost all potatoes are clones, as are all banana trees grown from root cuttings. For plants, this form of asexual reproduction (an individual copies its genetic material) is known as vegetative reproduction. This is how grass and other plants like strawberries grow and spread. Grass puts out underground shoots, and strawberries send out aboveground runners, both of which eventually form independent, new plants that are genetically identical to the original or parent plant. Most bacteria are also natural clones since they reproduce by a process called binary fission in which they basically split in two, making a pair of identical cells.

Besides these natural types of cloning, a recently developed artificial type of cloning occurs when a segment of deoxyribonucleic acid (DNA) is duplicated outside the body of a plant or animal. Advances with this type of research in which exact copies of DNA segments were made eventually led to scientists being able to clone a complex organism. For example, in 1968, the English biologist John Gurdon cloned a frog by replacing the nucleus of a frog egg cell with the nucleus (a cell's control center) of a cell from another frog's embryo. The egg cell matured into an exact identical twin of the tadpole embryo. Following this success, biologists attempted to clone mice and white rats, but most of the clones did not survive. Cloning mammals proved to be even more difficult and inefficient, with most attempts failing because the cell taken from the embryo was too mature. Its cells had already begun to specialize, as some started making cells for different organs and others making skin cells and limb cells. Overall, it proved very difficult to obtain a mammal embryo cell in its earliest stages of development.

This problem was solved on July 5, 1996 when a sheep named Dolly was born in Edinburgh, Scotland. In a dramatic breakthrough, the Scottish embryologist Ian Wilmut was able to clone a mammal from a cell taken not from an embryo but from an adult. His startling success, announced when Dolly was about seven months old, was achieved by Wilmut's unique method of "starving" a cell's nucleus which made it revert back to an earlier stage of development. First, Wilmut took unfertilized eggs from an adult female and removed all of its DNA. This left it an empty egg that could still support growth. He then took the udder cells from an adult sheep and raised them in a way designed to "turn off" their specialized genes. One of these donor cells was then fused electrically with the empty egg cell, and the artificially fertilized egg started to divide into an embryo. It was then transplanted into the womb of a sheep, and Dolly, the genetic twin of the animal who donated the udder cell and its own DNA, was eventually born.

IAN WILMUT

English embryologist (a person specializing in the study of the early development of living things) Ian Wilmut (1944– ) produced the first mammal to be cloned from an adult animal. This biological breakthrough meant that future cloned animals might be used to produce large quantities of proteins needed for making certain drugs. It also suggested that these animals might provide a safer organ transplant source for humans.

Ian Wilmut was born in Hampton Lucey, England, the son of a mathematics teacher. He became fascinated with embryology while earning a degree in agricultural science at the University of Nottingham in 1967. Wilmut continued his studies at Darwin College at Cambridge University in England and received a Ph.D. in animal genetic engineering in 1971. He then took a position at the Animal Breeding Research Station in Scotland, now known as the Roslin Institute. While at Darwin College, his dissertation topic was on techniques for freezing boar sperm, and in 1973 he created the first calf ever produced from a frozen embryo. Wilmut continued his research during the 1980s, always with the goal of cloning an animal in mind. A clone is the offspring that results from a form of asexual reproduction. This means that cloning involves only a single parent and does not require the exchange of sex cells from a male and female.

In 1990, Wilmut hired English cell biologist Keith Campbell to work in his cloning laboratory, and it was Campbell's idea that transplanted adult cells had not been working with embryo cells because the two were not "synchronized." Since cells go through specific cycles, regularly growing and dividing and making an entirely new package of chromosomes each time, Campbell argued that adult mammal cells had to be slowed down to be in synch with embryos. Wilmut and Campbell then pioneered a new technique of starving adult cells so they would eventually be in the same cycle as the embryos. Once they "turned off" the specialized adult genes (taken from the udder or milk gland of a six-year-old sheep) and made them act like embryo cells, they fused it with an unfertilized egg that had all of its genetic information-containing deoxyribonucleic acid (DNA) removed. After the artificially fertilized egg started to divide into an embryo, it was transplanted into the womb of a surrogate, or substitute, female sheep where it developed and grew, producing an offspring that was genetically identical to the animal that donated the cell.

Wilmut and Campbell, therefore, produced the cloned lamb named "Dolly" on July 5, 1996. As the first clone from an adult mammal, this successful experiment marked an achievement that some had thought would (or should) never be realized. It also set off a wave of discussion and debate about the implications and ethics of cloning. Naturally, that debate focused on the potential for cloning human beings. While Wilmut remained passionate about his achievement, he stated clearly that cloning a person is ethically unacceptable, and that the primary purpose of his work is to advance the development of drug therapies to combat certain life-threatening diseases. As an example of a health-related product developed from cloning, he offers the possibility of cloning an animal that produces the blood clotting factors that hemophiliacs are lacking. He also envisions organ transplants becoming plentiful and routine by means of inserting a human protein into a cloned animal that allows the animal organs to be more easily accepted by the human patient's body. Wilmut is aware of the ethical concerns many people have about cloning, and he stresses that it is very important to prevent any real misuse if humans are to gain any of cloning potential benefits.

The cloning of a mammal produced fear as well as praise among many people, as it raised the possibility of cloning a human being. Biologists tried to ease this fear by pointing out the medical advantages of being able to clone an animal that contains a certain human gene in its cells. They suggest that such animals could produce a particular enzyme needed by people whose bodies will not produce it, such as the blood-clotting enzyme thrombin, which hemophiliacs lack. However, as with all aspects of genetic engineering, cloning raises many issues with far-reaching social, legal, and ethical implications. These complex issues, in turn, raise many difficult questions, such as who decides what traits are desirable? Are biologists "playing God" by tampering with human DNA? And might a genetic mistake result in some sort of disaster in which a genetic monster like an uncontrollable plague is created?

[See alsoDNA; Genetic Engineering; Nucleic Acid; Reproduction, Asexual ]