What is animal cloning?
Asexual reproduction occurs in numerous bacteria, fungi, and plants, as well as some animals, leading to genetically identical offspring or clones. In addition, humans can assist in such reproduction. For instance, cuttings from plants generate thousands of replicates. Dividing some animals, such as earthworms or flatworms, allows them to regenerate. However, most vertebrates, including all mammals, reproduce sexually, requiring fertilization of an ovum by sperm. In such species, clones occur, as in the case of identical twins, when an embryo splits completely early in development. This process can be instigated artificially using microsurgical techniques to divide a harvested early-stage embryo and reimplanting the halves into surrogate dams (mothers). While this can be considered animal cloning, the term should be reserved for cloning from nonembryonic cells.
Animal cloning typically refers to mammals or other higher vertebrates and involves creating a duplicate animal starting from a differentiated cell. Although such a cell only has the ability to perform its specialized function, its nucleus retains all genetic information for the organism’s development. Animal cloning requires that such information be reprogrammed into an undifferentiated cell that can reinitiate the developmental process from embryo to birth and beyond.
In theory, the process, known as "somatic cell nuclear transfer (SCNT)" or "fusion cell cloning," is straightforward. It consists of taking a differentiated cell from an adult animal, inserting its diploid nucleus into a donor ovum whose own haploid nucleus has been removed, initiating embryonic development of this ovum, inserting the resultant embryonic mass into a receptive surrogate dam (in estrus) and allowing it to proceed to term. In practice, the technique is difficult and was thought to be impossible until 1997. It also appears fraught with species specificity. Various differentiated cells have been used as the starting source; mammary cells were used in the first case, while skin fibroblasts and cumulus cells are also often used. The preparation of the enucleated ovum is an important step. A limitation to cloning dogs appears to be the difficulty in determining when estrus will occur. The technique for inserting the nucleus is crucial, as is the conversion to the undifferentiated embryonic state. Transfer of the embryonic cells to a receptive surrogate dam is generally a well-developed technology, although more than three viable embryos are necessary to maintain pregnancy in pigs.
Furthermore, the genetic makeup of a putative clone must be verified, to ensure that it is indeed a replica of its progenitor and not an unintended offspring of either the donor of the ovum or the surrogate dam. DNA fingerprinting via microsatellite analysis at a number of polymorphic sites is an unambiguous way to establish its genetic identity.
Such a clone is not absolutely identical, because of mitochondrial differences and environmental effects. While the nuclear genome must be identical to its progenitor, the mitochondrial genome of the clone will invariably be different, because it comes from the ovum used. While mitochondria make a minor contribution to the total genetic makeup, they can influence phenotypic expression. In addition, the prenatal environment can affect some traits. Coat color and color pattern are characteristics that can be developmentally influenced; the first cloned cat was not an exact duplicate of its progenitor in coloration. Some behavioral features are also impacted during intrauterine development.
The first cloned animal was a sheep named Dolly. While she was the only live offspring generated from 277 attempts, her birth showed that animal cloning was possible. Shortly thereafter, mice and cattle were cloned. Reproducible cloning of mice is more difficult than imagined, whereas more cattle were cloned in the first five years after Dolly’s birth than any other species. Goats, pigs, cats, buffalo, gray wolves, and a camel were among the animals that were subsequently cloned.
Prominent among the problems with animal cloning is its inefficiency. Although this may not be surprising as the technology continues to evolve, SCNT still has a less than 10 percent success rate in the mid-2010s, according to the US Food and Drug Administration. Additionally, most cloned animals are larger than normal at birth, often requiring cesarian delivery, and some have increased morbidity and mortality. Some have had smaller telomeres and shorter lives. Dolly exhibited this trait and lived for only six years (although she was euthanized, she clearly would not have lived much longer)—half of the average life span. Conversely, some cloned mice do not exhibit shortened telomeres or premature aging, even through six consecutive cloned generations. Further research will establish whether these problems are inherent to cloning, are consequences of some aspect of the current procedure, or are attributable to the small numbers of cloned animals studied.
The benefits of animal cloning would involve duplicating particularly valuable animals. Livestock with highly valued production characteristics could be targets for cloning. However, the technique is likely to be most beneficial in connection with transgenesis, to replicate animals that yield a therapeutic agent in high quantities or organs suitable for transplantation into humans. Some researchers also hold out hope that cloning could one day help stabilize the populations of endangered species. If animal cloning can be made efficient and trouble-free, its potential benefits could be fully developed.
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