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Cloning Cloning is the process of creating an identical copy of an original. A clone in the biological sense, therefore, is a single cell (like bacteria, lymphocytes etc.) or multi-cellular organism that is genetically identical to another living organism. Sometimes this can refer to "natural" clones made either when an organism reproduces asexually or when two genetically identical individuals are produced by accident (as with identical twins), but in common parlance the clone is an identical copy by some conscious design. Also see clone (genetics). The term clone is derived from κλων, the Greek word for "twig". In horticulture, the spelling clon was used until the twentieth century; the final e came into use to indicate the vowel is a "long o" instead of a "short o". Since the term entered the popular lexicon in a more general context, the spelling clone has been used exclusively.
Cloning in biologyCloningCloning a gene means to extract a gene from one organism (for example by PCR) and insert it into a second organism (usually via a vector), where it can be used and studied. Cloning a gene sometimes can refer to success in identifying a gene associated with some phenotype. For example, when biologists say that the gene for disease X has been cloned, they mean that the gene's location and DNA sequence has been identified, although the ability to specifically copy the physical DNA is a side-effect of its identification. A related technique called subcloning refers to transferring a gene from one plasmid into another for further study. Cloned reproductive organs have been known to be able to grow back if damaged. Cellular
Cloning a cell means to derive a population of cells (a clonal population) from a single cell. This is an important in vitro procedure when the expansion of a single cell with certain characteristics is desired, for example in the production of gene-targeted ES cells. Most individuals began as a single cell (a zygote) and are therefore the result of clonal expansion in vivo. Cloning an organism broadly means to create a new organism with the same genetic information as a cell from an existing one. In a modern context, this can involve somatic cell nuclear transfer in which a cell of the organism to be cloned, with its nucleus containing the DNA, is transferred into an egg cell which has had its nucleus removed (currently, both the egg cell and the transferred nucleus must be from the same species). As the nucleus contains almost all of the genetic information of a lifeform, the "host" egg cell will develop into an organism with genetically identical nuclear DNA to the nucleus "donor". However, this process does not conserve the mitochondrial genome unless the nucleus and egg cell donor were the same individual. Thus, nuclear transfer clones are not clones in the strictest sense because the mitochondrial genome is not the same as that of the nucleus donor cell from which it was produced. This may have important implications for cross-species nuclear transfer in which nuclear-mitochondrial incompatibilities may lead to inviability. The first animal clone was a frog cloned by Thomas J. King and Robert W. Briggs in 1952. HorticulturalThe term clone is used in horticulture to mean all descendants of a single plant, produced by vegetative reproduction. Many horticultural varieties of plants are clones, having been derived from a single individual, multiplied by some process other than sexual reproduction. As an example, some European varieties of grapes represent clones that have been propagated for over two millennia. Other such examples are potatoes or bananas. Also grafting can be regarded as cloning, since all the shoots and branches coming from the graft are genetically a clone of a single individual. These are genuine examples of cloning in the broader biological sense, as they create genetically identical organisms by biological means, but this particular kind of cloning has not come under ethical scrutiny and is generally treated as an entirely different kind of operations. Natural clones
Cloning exists in nature in some species and is referred to as parthenogenesis. An example is the "Little Fire Ant," Wasmannia auropunctata, which is native to Central and South America but has spread throughout many tropical environments.[1] In this species, circumstantial evidence from microsatellite DNA suggests that both queens and males may reproduce clonally in one population in Suriname. Species clonedThe modern cloning techniques involving nuclear transfers have been successfully performed on several species. Land mark experiments in chronological order:
For a complete list see: List of animals that have been cloned. Ethical issues of cloningChristian views
Christian views of cloning are diverse and sometimes conflicting. Roman Catholicism and other conservative Christian denominations believe that the soul enters the body at the moment of conception when the sperm and egg unite. They feel harvesting cells for embryonic cloning is tantamount to live human experimentation and contrary to God's will. Further, they maintain that producing cloned zygotes that are unlikely to survive is tantamount to murder. Some Christian conservatives express concern that cloned embryos would have no soul, since it was, in their view, born outside of God's parameters. Other Christian traditions like the United Church of Christ do not believe a fertilized ovum constitutes a person. They believe that cloning can be conducted ethically in order to help treat disease and improve the public health. Libertarian views on the subject suggest that it is in a person's constitutional rights to conduct this process, similar to abortion. Health aspectsHowever, the success rate has been very low: Dolly was born after 276 failed attempts; 70 calves have been created from 9,000 attempts and one third of them died young; Prometea took 328 attempts, and, more recently, Paris Texas was created after 400 attempts. Notably, although the first clones were frogs, no adult cloned frog has yet been produced from a somatic adult nucleus donor cell. A surprising development to do with aging resulted from finds that Dolly was apparently subject to accelerated aging. Aging of this type is thought to be due to telomeres, regions at the tips of chromosomes which prevent genetic threads fraying every time a cell divides. Over time telomeres get worn down until cell-division is no longer possible - this is thought to be a cause of aging. However, when researchers cloned cows they appeared to age more slowly than expected. Analysis of the cow's telomeres showed they had not only been 'reset' to birth-length, but they were actually longer - suggesting these clones would live longer life spans than normal cows (but many have died young after excessive growth). Researchers think that this could eventually be developed to reverse aging in humans, provided that this is based chiefly on shortening of telomeres. Although some work has been performed on telomeres and aging in nuclear transfer clones, the evidence is contradictory and does not support any generalizable link. Dolly died in the year of 2003. Therapeutic cloning is the procedure for creating stem cells genetically compatible with the patient. Therapeutic cloning might provide a way to grow organs in host carrier, which become completely compatible with the original. Host carrier growing poses a risk of trans-species diseases if the host is of a different species (e.g. a pig.) In human beings, this is a highly controversial issue, as it involves creating human embryos in vitro and then destroying them to obtain multipotent embryonic stem cells. Human cloning
Human cloning is the creation of a genetically identical copy of an existing, or previously existing human or growing cloned tissue from that individual. The term is generally used to refer to artificial human cloning; human clones in the form of identical twins are commonplace, with their cloning occurring during the natural process of reproduction. Cloning extinct speciesCloning, or more precisely, the reconstruction of functional DNA from extinct species has, for decades, been a dream of some scientists. The possible implications of this were dramatized in the novel by Michael Crichton and high budget Hollywood thriller, "Jurassic Park". In real life, one of the most anticipated targets for cloning was once the Woolly mammoth, but attempts to extract DNA from frozen mammoths have been unsuccessful, though a Japanese team is currently working toward this goal. In 2000, a cow named Bessie gave birth to a cloned Asian gaur, an endangered species, but the calf died after 2 days; the attempt to clone a banteng was more successful and provided hope that similar techniques (using surrogate mothers of another species) might be used to clone extinct species; in anticipation of this possibility, the last bucardo, a Pyrenean ibex, was frozen immediately after it died (from illness after birth). Researchers are also considering cloning endangered species such as the giant panda, ocelot, and cheetah[7]. See the discussion under "Dolly" for a discussion of the promises and limitations of this approach. In 2002, geneticists at the Australian Museum announced that they had replicated DNA of the Thylacine (Tasmanian Tiger), extinct about 65 years previous, using polymerase chain reaction (PCR)[8]. However, on February 15, 2005 the museum announced that it was stopping the project after tests showed the specimens' DNA had been too badly degraded by the (ethanol) preservative. Most recently, on May 15, 2005, it was announced that the project would be revived, with new participation from researchers in New South Wales and Victoria. One of the continuing obstacles in the attempt to clone extinct species is the need for nearly perfect DNA. Cloning from a single specimen could not create a viable breeding population in sexually reproducing animals. Furthermore, even if males and females could be cloned, the question would remain open if they would be viable at all in the absence of parents that teach or show them natural behavior. Essentially, even if cloning an extinct species would succeed - it must be considered that cloning still is an experimental technology that succeeds only by chance -, it is far more likely than not that any resulting animals, even if they were healthy, would be little more than curios or museum pieces. Most conservation biologists are rather vehemently opposed to cloning and consider it a smokescreen fit for generating headlines, but detrimental to conservation success, as funds needed to preserve habitat and wild populations threaten to be diverted to such cloning projects and eventually might even cause the extinction of species in a wild state; the rule-of-thumb in animal conservation is that conservation attempts in captivity are not to be undertaken on a standalone basis if it is still feasible to conserve habitat and viable wild populations. The banteng cloning project was an exception, as the animal cloned was a distinct genetic lineage and the value of preserving this piece of genetic diversity of an already inbred species outweighed the uncertainties. Dolly The Sheep
Dolly (5 July 1996 – 14 February 2003), an ewe, was the first mammal to have been successfully cloned from an adult cell. She was cloned at the Roslin Institute in Scotland and lived there until her death when she was 6. Her birth was announced on 22 February 1997. Ian Wilmut is the leader of the research team who created Dolly. The name "Dolly" came from a suggestion by Jesse Haase who helped with her birth, in honour of Dolly Parton, because it was a mammary cell that was cloned. The technique that was made famous by her birth is somatic cell nuclear transfer, in which a non-reproductive cell containing a nucleus is placed in a de-nucleated ovum (which then develops into a fetus). When Dolly was cloned in 1996 from a cell taken from a six-year-old ewe, she became the centre of much controversy that still exists today. On 9 April 2003 her stuffed remains were placed at Edinburgh's Royal Museum, part of the National Museums of Scotland. Technical HurdlesCloning is quite inefficient and usually there are over 600 to 1000 nuclear transfers before one is able to grow into a stem cell. This inefficiency is thought to be due to genetic imprinting in the cloned adult cell that interferes with the correct gene expression in the embryo. Even those animals that are successfully cloned are not as healthy as the original animal. For example, Dolly had arthritis and sign of premature aging. see methylation and epigenetic PublicationsCloning and Stem Cells, published by Mary Ann Liebert, Inc., publishes peer-reviewed research papers on the remarkable new opportunities in medicine, biology, and agriculture that arise from the demonstration of far greater than expected developmental plasticity in mammalian cells. Papers cover all aspects of cloning along with the culture and differentiation of stem cells from all stages of development from embryo to adult. Cloning in fictionCloning has been widely explored in science-fiction.
Two additional fictional works on cloning: The Third Twin, by Ken Follett. Research scientist discovers that her boss has created dangerous, violent clones in a failed attempt to create "super-soldiers." A Number, by Caryl Churchill (2003). Play about a man who creates a clone to replace a son he's dissatisfied with, and what happens when they discover each other's existence 35 years later. Cloning is also used in the 2005 movie "Isolation", a horror film about the effects of cattle cloning, where Large Offspring Syndrome goes haywire. End NotesExternal links and references
This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia Encyclopedia article "Cloning" |
