Reproductive cloning is a technology used to generate an animal that has the same nuclear DNA as another currently or previously existing animal. Dolly the sheep was created by reproductive cloning technology. In a process called "somatic cell nuclear transfer" (SCNT), scientists transfer genetic material from the nucleus of a donor adult cell to an egg whose nucleus, and thus its genetic material, has been removed. The reconstructed egg containing the DNA from a donor cell must be treated with chemicals or electric current in order to stimulate cell division. Once the cloned embryo reaches a suitable stage, it is transferred to the uterus of a female host where it continues to develop until birth.
Dolly or any other animal created using nuclear transfer technology is not truly an identical clone of the donor animal. Only the clone's chromosomal or nuclear DNA is the same as the donor. Some of the clone's genetic materials come from the mitochondria in the cytoplasm of the enucleated egg. Mitochondria, which are organelles that serve as power sources to the cell, contain their own short segments of DNA, although this is only 0.01% of the total DNA. Acquired mutations in mitochondrial DNA are believed to play an important role in the growing process.
Mutations also occur with every cell division so no two cells in an individual are identical. Thus, nuclear transfer clones from different maternal lineages 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 death.
The modern cloning techniques involving nuclear transfer have been successfully performed on several species. Landmark experiments in chronological order:
" Tadpole: (1952) Many scientists questioned whether cloning had actually occurred and unpublished experiments by other labs were not able to reproduce the reported results.
" Carp: (1963) In China, embryologist Tong Dizhou cloned a fish. He published the findings in an obscure Chinese science journal which was never translated into English.
" Sheep: (1996) From early embryonic cells by Steen Willadsen. Megan and Morag cloned from differentiated embryonic cells in June 1995 and Dolly the sheep in 1997.
" Rhesus Monkey: Tetra (female, January 2000) from embryo splitting
" Cattle: Alpha and Beta (males, 2001) and (2005) Brazil
" Cat: CopyCat "CC" (female, late 2001), Little Nicky, 2004, was the first cat cloned for commercial reasons
" Mule: Idaho Gem, a john mule born 2003-05-04, was the first horse-family clone.
" Horse: Prometea, a Haflinger female born 2003-05-28, was the first horse clone.
For a complete list see: List of animals that have been cloned.
The success rate of cloning has been low: Dolly the sheep was born after 277 eggs were used to create 29 embryos, which only produced three lambs at birth, only one of which lived, Dolly. Seventy 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.
There were early claims that Dolly the Sheep had accelerated aging. Aging of this type is thought to be due to the shortening of telomeres, regions at the tips of chromosomes which prevent genetic threads from 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, subsequent studies showed that, if anything, Dolly's telomere were longer than normal. Dolly died in the year of 2003. Ian Wilmut said that Dolly's early death had nothing to do with cloning but with a respiratory infection common to lambs raised like Dolly.
Consistent with Dolly's telomeres being longer, analysis of the telomeres from cloned cows showed that they were also longer. This suggests clones could live longer life spans although many 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 the shortening of telomeres. Although some work has been performed on telomeres and aging in nuclear transfer clones, the evidence is at an early stage.
Human cloning is the creation of a genetically identical copy of an existing, or previously existing human, by 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.
Human cloning is amongst the most controversial forms of the practice. There have been numerous demands for all progress in the human cloning field to be halted. One of the most ethically questionable problems with human cloning is farming of organs from clones. For example, many believe it is unethical to use a human clone to save the life of another. In this scenario, the cloned human would be euthanized so that the vital organs could be harvested. This process of renewing the body's organs would potentially increase the life expectancy of a human by 50 years. Some people have considered the idea of growing organs separately from a human organism - in doing this, a new organ supply could be established without the moral implications of harvesting them from human organisms. Research is also being done on the idea of growing organs that are biologically acceptable to the human body inside of other organisms, such as pigs or cows, then transplanting them to humans. This practice is still morally questionable, but arguably less so than the process of harvesting said organs from a cloned human being.
The cloning describedabove is reproductive cloning, not to be confused with research cloning in which only parts (such as an organ) are cloned using genetic material from a patient's tissues.
Ethical issues of human cloning
Although the practice of cloning organisms has been widespread for several thousands of years in the form of horticultural cloning, the recent technological advancements that have allowed for cloning of animals (and potentially humans) have been highly controversial. Many religious groups oppose all forms of cloning, including the potentially life-saving cloning of individual organs, on the grounds that life begins at conception. Concerns also exist regarding the protection of the identity of the individual and the right to protect one's genetic identity. In addition, if technology eventually does allow for successful cloning of humans, prejudice may develop against clones, as if they were a "lesser" form of human being. Such prejudice could force clones into a kind of slavery or caste system. The possible social implications of an artificial human production scheme were famously explored in the novel Brave New World.
1. Vogel, Gretchen (2000). "In Contrast to Dolly, Cloning Resets Telomere Clock in Cattle". Science 288: 641.
2. Pence, Gregory E. (1998). Who's Afraid of Human Cloning?. Rowman & Littlefield.
3. Heidi B. Perlman. "Scientists Close on Extinct Cloning", Associated Press, 2000-10-08.
4. Pence, Gregory E. (2005). Cloning After Dolly: Who's Still Afraid?
5. Holloway, Grant. "Cloning to revive extinct species", CNN.com, 2002-05-28.