There are three different types of artificial cloning: gene cloning, reproductive cloning and therapeutic cloning. Gene cloning produces copies of genes or segments of DNA. Reproductive cloning produces copies of whole animals.
Therapeutic cloning produces embryonic stem cells for experiments aimed at creating tissues to replace injured or diseased tissues. Gene cloning, also known as DNA cloning, is a very different process from reproductive and therapeutic cloning. Reproductive and therapeutic cloning share many of the same techniques, but are done for different purposes.
Researchers routinely use cloning techniques to make copies of genes that they wish to study. The procedure consists of inserting a gene from one organism, often referred to as "foreign DNA," into the genetic material of a carrier called a vector. Examples of vectors include bacteria, yeast cells, viruses or plasmids, which are small DNA circles carried by bacteria. After the gene is inserted, the vector is placed in laboratory conditions that prompt it to multiply, resulting in the gene being copied many times over.
In reproductive cloning, researchers remove a mature somatic cell , such as a skin cell, from an animal that they wish to copy. They then transfer the DNA of the donor animal's somatic cell into an egg cell, or oocyte, that has had its own DNA-containing nucleus removed. Researchers can add the DNA from the somatic cell to the empty egg in two different ways. In the first method, they remove the DNA-containing nucleus of the somatic cell with a needle and inject it into the empty egg. In the second approach, they use an electrical current to fuse the entire somatic cell with the empty egg.
In both processes, the egg is allowed to develop into an early-stage embryo in the test-tube and then is implanted into the womb of an adult female animal.
Ultimately, the adult female gives birth to an animal that has the same genetic make up as the animal that donated the somatic cell. This young animal is referred to as a clone. Reproductive cloning may require the use of a surrogate mother to allow development of the cloned embryo, as was the case for the most famous cloned organism, Dolly the sheep. Over the last 50 years, scientists have conducted cloning experiments in a wide range of animals using a variety of techniques.
In , researchers produced the first genetically identical mice by splitting mouse embryos in the test tube and then implanting the resulting embryos into the wombs of adult female mice. Shortly after that, researchers produced the first genetically identical cows, sheep and chickens by transferring the nucleus of a cell taken from an early embryo into an egg that had been emptied of its nucleus.
It was not until , however, that researchers succeeded in cloning the first mammal from a mature somatic cell taken from an adult animal. After attempts, Scottish researchers finally produced Dolly, the lamb from the udder cell of a 6-year-old sheep.
Two years later, researchers in Japan cloned eight calves from a single cow, but only four survived. Besides cattle and sheep, other mammals that have been cloned from somatic cells include: cat, deer, dog, horse, mule, ox, rabbit and rat.
In addition, a rhesus monkey has been cloned by embryo splitting. Despite several highly publicized claims, human cloning still appears to be fiction. There currently is no solid scientific evidence that anyone has cloned human embryos. In , scientists in South Korea claimed to have successfully cloned a human embryo, but said the experiment was interrupted very early when the clone was just a group of four cells.
In , Clonaid, part of a religious group that believes humans were created by extraterrestrials, held a news conference to announce the birth of what it claimed to be the first cloned human, a girl named Eve.
However, despite repeated requests by the research community and the news media, Clonaid never provided any evidence to confirm the existence of this clone or the other 12 human clones it purportedly created. In , a group led by Woo-Suk Hwang of Seoul National University in South Korea published a paper in the journal Science in which it claimed to have created a cloned human embryo in a test tube. However, an independent scientific committee later found no proof to support the claim and, in January , Science announced that Hwang's paper had been retracted.
From a technical perspective, cloning humans and other primates is more difficult than in other mammals. One reason is that two proteins essential to cell division, known as spindle proteins, are located very close to the chromosomes in primate eggs. Consequently, removal of the egg's nucleus to make room for the donor nucleus also removes the spindle proteins, interfering with cell division.
In other mammals, such as cats, rabbits and mice, the two spindle proteins are spread throughout the egg. So, removal of the egg's nucleus does not result in loss of spindle proteins.
In addition, some dyes and the ultraviolet light used to remove the egg's nucleus can damage the primate cell and prevent it from growing. One genetic crossover is equivalent to 50 percent recombination between loci on opposite sides of the cross over. If this concept is extended over the entire karyotype, the human genetic individuality would be self-explanatory.
Crossing over and fertilization are two important events, which lead to unique characteristics of the individual with the possibility of acquiring characteristics from six sources, the maternal and paternal grandmothers and grandfathers, and the parents.
In cloned individual, the genetic contribution would essentially be confined to the donor. If an adult-cell is cloned, the result would be an identical twin of the donor without any crossing over and mixing of genetic characteristics of the parents of the donor. If nuclear transfer is used, there would be some genetic contribution from the mitochondria of the recipient cell besides the contribution from the donor nucleus.
But the effect of this is yet unknown. However, in both circumstances the nuclear genetic contribution would essentially be confined to the donor genome. This would certainly rob off the child of the unique characteristics, a natural child possesses. While it might not be apparent with cloning a handful of individuals, the question is what would happen in case of mass production?
Sudipta Paul is the sole Author who contributed to the study including participation in study design, execution, analysis, manuscript drafting and critical discussion. This is an open access article distributed under the terms of the, which permits unrestricted use, distribution, and build upon your work non-commercially. Withdrawal Guidlines. Publication Ethics. Withdrawal Policies Publication Ethics. Keywords: human cloning, evolution, crossing over. Methods of cloning There are essentially two techniques.
Cloning and unique individual Crossing over and fertilization are two important events, which lead to unique characteristics of the individual with the possibility of acquiring characteristics from six sources, the maternal and paternal grandmothers and grandfathers, and the parents.
Stephenson J. Threatened bans on human cloning research could hamper advances. First principles in cloning. The Lancet. Taylor R. Super humans. The nation's top scientists from The National Academies of Science and National Institutes of Health, as well as numerous Nobel Laureates attest to the scientific value of this research. A February, report from the National Academies of Science concluded that while reproductive cloning is unsafe and should be banned, therapeutic cloning has sufficient scientific potential that it should be allowed to continue.
Stem cell research will help scientists learn how to develop cells and tissue to cure disease. Over many years, scientists have demonstrated that they may learn how to induce these cells to differentiate into many different cell types.
Accomplishing that would enable scientists to create new, healthy cells and tissue for transplantation to replace damaged or dead tissue. But one major problem with developing disease therapies with stem cells is the body's immune response system. When cells, including donated organs, tissues or blood, are transplanted or transfused, the recipient's body mounts a rejection response, attacking these cells as foreign.
If a patient's own cells were the source of stem cells used to create therapeutic cells or tissues, it is believed that immunological rejection could be avoided since the cells and tissues would genetically match his own.
Therapeutic cloning could allow an individual's own cells to be used to treat or cure that person's disease, without risk of introducing foreign cells that may be rejected. Thus, cloning is vital to realizing the potential of stem cell research and moving it from the lab into the doctor's office.
SCNT involves removing the nucleus of an egg cell, replacing it with the material from the nucleus of a "somatic cell" such as a skin cell and stimulating this cell to begin dividing.
This egg cell is never fertilized by sperm , and the genetic material within the cell is virtually identical to the genetic material extracted from the skin or other cell. Once the cells begins dividing, stem cells can be extracted from it days later, just as they can be extracted from embryos created through in vitro fertilization. These stem cells - or the specialized cells derived from them - will be a genetic match to the patient.
Therefore, the promise of SCNT is that the patient's body would accept these cells after transplantation. By using SCNT, scientists hope to understand how the protein factors in the egg cell cause these already specialized somatic cells to become stem cells.
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