Clone and Cloning
Clone and Cloning
A clone is a cell that is identical to the cell it was derived from. This identity is complete, extending from the cell’s genetic material (DNA and RNA) to the molecules that make up the cell.
There are three types of cloning. One method, gene cloning, utilizes copying fragments of DNA for easier manipulation and study. Another cloning method involves producing genetically identical animals through a process called twinning. The final cloning method involves producing an organism through a nuclear transfer of genetic material from adult cell into an egg. Cloning is ancient. Before the existence of the deliberategenetic engineering at the molecular level, people cloned plants by grafts and stem cuttings. Modern-day cloning involving molecular techniques is a relatively recent scientific advance that sprang from the demonstration in the mid-1970s that targeted genetic material could be retrieved from one organism and inserted into another organism to become functional. Now, cloning is at the forefront of modern biology. Cloning has many promising applications in medicine, industry, conservation, and basic research. It is also the subject of much contention and debate.
Humans have manipulated plant asexual reproduction through methods like grafting and stem cuttings for more than 2, 000 years. The modern era of laboratory cloning began in 1958 when carrot plants were cloned from mature single cells placed in a nutrient culture containing hormones. The first cloning of animal cells took place in 1964 when John Gurdon took the nuclei from intestinal cells of toad tadpoles and injected them into unfertilized eggs whose nuclei containing the original parents’ genetic information had been destroyed with ultraviolet light. When the eggs were incubated, Gurdon found that 1–2% of the eggs developed into fertile, adult toads.
The first successful cloning of mammals was achieved nearly 20 years later when scientists in both Switzerland and the United States successfully cloned mice using a method similar to Gurdon’s approach; but their method required one extra step. After the nuclei were taken from the embryos of one type of mouse, they were transferred into the embryos of another type of mouse who served as a surrogate mother that went through the birthing process to create the cloned mice. The cloning of cattle livestock was achieved in 1988 when nuclei from embryos of prize cows were transplanted to unfertilized cow eggs whose own nuclei had been removed.
In 1997, Scottish scientists cloned a sheep named Dolly, using cells from the mammary glands of an adult sheep and an egg cell from which the nucleus had been removed. This was the first time adult cells, rather than embryonic cells, had been used to clone a mammal. Since then, mice, cattle, goats, and other mammals have been cloned by similar methods. Some of these clones have been genetically altered so that the animals can produce drugs used in human medicine. Scientists are trying to clone organs for human transplant and may soon be able to clone human beings.
In 2001, scientists from Advanced Cell Technology cloned the first endangered animal, a bull gaur (a wild ox from Asia). The newborn died after two days due to infection. In 2003, clones of a species of wild cattle, bantengs, were produced from cellular material stored more than 20 years In addition, an endangered mouflon was cloned using somatic cells from post-mortem samples. Since 2003 other species including cats, mules, and horses have been used to produce clones.
The cloning of specific genes can provide large numbers of copies of the gene for use in genetics, medical research, and systematics. Gene cloning begins by separating a specific length of DNA that contains the target gene. This fragment is then placed into another DNA molecule called the vector, which is then called a recombinant DNA molecule. The recombinant DNA molecule is used to transport the gene into a host cell, such as a bacterium or yeast, where the vector DNA replicates independently of the nuclear DNA to produce many copies, or clones, of the target gene. Recombinant DNA can also be introduced into plant or animal cells, but if the cells are to produce a particular protein (e.g., hormone or enzyme) for a long time, the introduced DNA molecule has to integrate into the nuclear DNA.
Blastomeres— Individual embryonic cells.
Cell cycle— A cycle of growth and cellular reproduction that includes nuclear division (mitosis) and cell division (cytokinesis).
Chromosomes— he structures that carry genetic information in the form of DNA. Chromosomes are located within every cell and are responsible for directing the development and functioning of all the cells in the body.
DNA— Deoxyribonucleic acid; the genetic material in a cell.
Embryo— The earliest stage of animal development in the uterus before the animal is considered a fetus (which is usually the point at which the embryo takes on the basic physical form of its species).
Gene— A discrete unit of inheritance, represented by a portion of DNA located on a chromosome. The gene is a code for the production of a specific kind of protein or RNA molecule, and therefore for a specific inherited characteristic.
Genetic engineering— The manipulation of genetic material to produce specific results in an organism. Genetics—The study of hereditary traits passed on through the genes.
Heredity— Characteristics passed on from parents to offspring.
Hybrid— The offspring resulting from combination of two different varieties of plants.
Nucleus (plural nuclei)— The part of the cell that contains most of its genetic material, including chromosomes and DNA.
In nature, the simple organisms such as bacteria, yeast, and some other small organisms use cloning (asexual reproduction) to multiply, mainly by budding.
The cloning of animal cells in laboratories has been achieved mainly through the methods of twinning and nuclear transfer. Twinning occurs when an early stage embryo is divided in vitro and inserted into surrogate mothers to develop to term. Nuclear transfer relies on the transfer of the nucleus into a fertilised egg from which the nucleus was removed. The progeny from the first procedure is identical to each other while being different from their parents. In contrast progeny from the second procedure share only the nuclear DNA with the donor, but not mitochondrial DNA and in fact it is not identical to the donor.
In 1993, the first human embryos were cloned using a technique that placed individual embryonic cells (blastomeres) in a nutrient culture where the cells then divided into 48 new embryos. These experiments were conducted as part of some studies on in vitro (out of the body) fertilization aimed at developing fertilized eggs in test tubes, which could then be implanted into the wombs of women having difficulty becoming pregnant. However, these fertilized eggs did not develop to a stage that was suitable for transplantation into a human uterus.
Cloning cells intially held promise to produce many benefits in farming, medicine, and basic research. In agriculture, the goal is to clone plants containing specific traits that make them superior to naturally occurring plants. For example, in 1985, field tests were conducted using clones of plants whose genes had been altered in the laboratory (by genetic engineering) to produce resistance to insects, viruses, and bacteria. New strains of plants resulting from the cloning of specific traits could also lead to fruits and vegetables with improved nutritional qualities and longer shelf lives, or new strains of plants that can grow in poor soil or even under water. A cloning technique known as twinning could induce livestock to give birth to twins or even triplets, thus reducing the amount of feed needed to produce meat.
In medicine, gene cloning has been used to produce vaccines and hormones, for example: insulin for treating diabetes and of growth hormones for children who do not produce enough hormones for normal growth. The use of monoclonal antibodies in disease treatment and research involves combining two different kinds of cells (such as mouse and human cancer cells), to produce large quantities of specific antibodies, which are produced by the immune system to fight off disease.
Recent years revealed that some cloned animals suffer from age-related diseases and die prematurely. Although, clones from other species still appear healthy, mice cloned using somatic cells have a higher than expected death rate from infections and hepatic failure.
Plagued with a chronic and progressive lung disease, veterinarians were forced to humanly euthanize Dolly in February 2003. Dolly lived a shorter lifespan than is typical ofsheep (normally 11 to 12 years) and developed other conditions such as chronic arthritis) much earlier than would be normally be expected.
Dolly’s seemingly fragile health, along with more generalized fears of premature aging in cloned animals, renewed fears first raised by a study in 1999 that Dolly’s telomeres were shorter than normal. Telomeres are the physical ends of eukaryotic chromosomes that function in the replication and stabilization of the chromosomes. Telomeres are synthesized by the enzyme telomerase. The enzyme may be important in determining the effective lifetime of a chromo-some and so may act as a biological clock for the cell. According to the telomere theory of aging, during DNA synthesis, DNA polymerase fails to replicate all of the nucleic acids resulting in shortened telomeres—and hence shortened chromosomes—with each successive generation of cell division. Eventually, the cell will no longer divide and after enough critical regions have been deleted, the cell cycle will arrest and the cell will die.
Because telomerase is not active all the time, nor is it found in every cell of the body, the genetic regulation of telomerase is under intense study. Researchers have discovered that if the action of telomerase is interrupted, the telomere will abnormally shorten and thus accelerate the general aging process of the cell.
At a minimum, cloning eliminates genetic variation and thus, can be detrimental in the long term, leading to inbreeding and increased susceptibility to diseases. Although cloning also holds promise for saving certain rare breeds of animals from extinction, for some of them, finding the surrogate mothers can be a challenge best illustrated by failed trials with cloning of pandas.
Despite the benefits of cloning and its many promising avenues of research, certain ethical questions concerning the possible abuse of cloning have been raised. At the heart of these questions is the idea of humans tampering with life in a way that could harm society, either morally, or in a real physical sense. Despite these concerns, there is little doubt that cloning will continue to be used. Cloning of particular genes for research and production of medicines is usually not opposed, gene therapy is much more controversial, while cloning of human beings is nearly uniformly opposed. Human cloning was banned in most countries and even the use of human embryonic stem cells is being reviewed in many countries. On the other hand, cloning plants or animals will probably continue.
Brown, Terry. Gene Cloning and DNA Analysis. Boston: Blackwell Publishing, 2006.
Caplan, Arthur and Glenn McGee, eds. The Human Cloning Debate. Berkeley: Berkeley Hills Books, 2006. McGee, Glenn. The Human Cloning Debate. Berkeley: Berkeley Hills Books, 2002.
Scientific American. Understanding Cloning. New York: Warner Books, 2002.
Loi, Pasqualino, Grazyna Ptak, Barbara Barboni, Josef Fulka Jr., Pietro Cappai, and Michael Clinton, “Genetic Rescue of an Endangered Mammal by Cross-species Nuclear Transfer Using Post-mortem Somatic Cells.” Nature Biotechnology (October 2001):962–964
Ogonuki, Narumi, et al. “Early Death of Mice Cloned from Somatic Cells.” Nature Genetics (11 February 2002):253-254