Cloning refers to the process of generating genetically identical copies of life forms, whether DNA fragments, genes, cells, or whole organisms such as plants, animals, or human beings. Although known primarily as a 20th-century biotechnology, “cloning” has long existed, both naturally and artificially. Asexual reproduction, which occurs via cell division (i.e., mitosis) and where offspring arise from a single parent (through processes such as budding, sporulation, fragmentation, and regeneration), is a natural form of cloning that is found in certain plants, microorganisms, and creatures such as sea anemones, starfish, flatworms, and some lizards. Twinning, or polyembryony (the development of two or more embryos from a single fertilized egg), is another example of natural cloning that is associated with sexual reproduction. Twinning occurs when a fertilized egg (a zygote) attempts to develop into a multicellular embryo through cell division. Instead of remaining part of the same embryo, however, these cells separate completely and begin to grow into full, but genetically identical organisms of their own. Thus, natural human clones exist in the form of identical twins.
Cloning also can be accomplished artificially. Three primary methods of artificial cloning include molecular cloning, embryonic cloning, and nuclear transplantation. In molecular cloning, a host organism (usually a single cell bacterium) is used to replicate human DNA sequences and genes. This provides scientists with unlimited copies of genetic material that can be used for basic research and the development of medical treatments.
Embryonic cloning is also sometimes referred to as therapeutic cloning. Similar to twinning, in embryonic cloning stem cells are extracted from young embryos (blastocysts, which are fertilized eggs about five days old). Because stem cells are unspecialized cells with the ability to develop into any type of cell, therapeutic cloning offers the potential for advances in regenerative medicine, including the manufacture of genetically compatible replacement cells and tissues for people in need of new organs or suffering from degenerative diseases. Bioethical concerns exist regarding therapeutic cloning, however, primarily because the harvesting of stem cells from blastocysts destroys the embryo.
Nuclear transplantation was first proposed by German embryologist Hans Spemann in his 1938 book, Embryonic Development and Induction, which was based on animal experiments he conducted throughout the early 1900s. In nuclear transplantation, the nucleus of a host cell is replaced with the nucleus from a donor cell. Because the nucleus contains the complete genetic material of an organism (except for mitochondrial DNA, which exists outside of the nucleus), the host cell becomes a clone of the donor.
Reproductive cloning, or the duplication of existing organisms, is often associated with somatic cell nuclear transfer (SCNT). This involves replacing the nucleus of an egg (an oocyte) with that taken from a body (somatic) cell of an adult. The new cell, akin to a fertilized egg in that it possesses a complete set of genes, is implanted in a surrogate and allowed to develop and grow. The ensuing offspring is a clone or a twin (save for mitochondrial DNA) of the adult.
Perhaps, the most famous case of a successful SCNT clone is Dolly, a sheep cloned in the mid-1990s by Ian Wilmut and colleagues at the Roslin Institute in Scotland using the nucleus of a mammary cell from one sheep and an oocyte from another. A third sheep was employed as a surrogate that carried Dolly to term. Dolly's existence was announced to the world on February 27, 1997. Prior to Dolly, other animals had been cloned, including frogs, fish, and mice. Since Dolly, cattle, camels, goats, deer, horses, mules, pigs, monkeys, cats, and dogs, among others, have been added to the list.
In 2003, a Québec-based sect known as the Raëlians claimed it had cloned five human beings. Clonaid, a Bahamas-based company founded in 1997 by Raëlians leader and former race-car driver Claude Vorilhon (aka Raël), alleged that the first cloned baby, “Eve,” was born on December 26, 2002, to a Florida woman and that four more cloned children had been born in the following six weeks. There was and continues to be no evidence for the validity of these claims.
In response to advances in cloning technologies and (potential) attempts to clone a human being, state governments and international agencies have drafted and/or passed laws or declarations limiting or banning cloning. Existing anticloning laws vary, some of which ban cloning outright, bar reproductive but not therapeutic cloning, regulate cloning, and/or prohibit the use of public funding for cloning. In 2005, the UN General Assembly approved the United Nations Declaration on Human Cloning.
While the ethics of both therapeutic and reproductive cloning remain important subjects in science, medicine, and the law, the power to replicate genes, cells, and even entire individual beings allows the technique of cloning to be seen as a potential defense against death and even a form of immortality.
Cloning is the process of creating a genetic duplicate of a previous organism. Cloning is common in nature as a method of reproduction. As...
Dolly, born in 1996, achieved notoriety by being the first mammal to be cloned from the nucleus of a fully differentiated cell, taken from the...
An important consequence of the Boyer-Cohen experiments was their possible adaptation for cloning. Cloning is the process by which an exact copy...