Introduction
A clone is an identical genetic copy of a biological entity. Cloning occurs in nature, for example if
single celled organisms such as bacteria reproduce through asexual reproduction. In mammals,
identical twins are created when a fertilized egg splits, giving them almost identical DNA to each
other, but different DNA from either parent. Clones do not always look identical however, given
different ways that genes can be interpreted and also the role the environment plays in how an
organism develops. Artificial cloning can refer to overlapping but separate technologies that are
focused on a range of materials such as genes, cells, organs, and entire organisms. These
technologies are, firstly, recombinant DNA cloning—reproductive cloning that is the cloning process
most detailed in the press—and secondly, therapeutic cloning.
Words to Know
Plasmid
A circular strand of DNA molecules capable of self-replication that operates separately from
normalchromosomes.
Somatic
Pertaining to the corporeal; the cells of the body that make up tissues and organs, distinct from
those involved in reproduction.
Somatic Cell NUCLEAR Transfer (SCNT)
SCNT involves the use of adult cells to provide the nucleus in the cloning process.
Transgenic
An organism or cell into which the genes of another species have been combined.
Historical Background and Scientific Foundations
Although the term clone was not used until 1963 in a speech “Biological Possibilities for the Human
Species of the Next Ten-Thousand Years” by British-born Indian geneticist J. B. S. Haldane (1892–
1964), investigation into genetics had begun much earlier with the work of August Weismann (1934–
1914) in the late 1880s. Weismann proposed that cell differentiation would reduce the genetic
information contained within a cell. This theory pervaded until 1902 when the German embryologist
Hans Spelmann (1869–1941) showed how split salamander embryos could still grow to adulthood. In
1938, Spelmann stated that the way forward for this area of research should be by inserting the
nucleus of a differentiated cell into an enucleated egg.
In 1952 the tadpole of a northern leopard frog was the first in a long line of animals to be cloned from
embryonic cells. It was followed in 1953 by the discovery by molecular biologists James D. Watson
(1928–) and Francis Crick (1916-2004) of the structure of DNA at Cambridge's Cavendish Laboratory.
Two decades later, in 1979, researchers cloned mice by splitting mouse embryos (in a similar fashion
to how twins occur in nature) and implanting the split embryos into surrogate mice. However the first
animal to be cloned by somatic (mature) cell nuclear transfer (SCNT) was Dolly the sheep, in 1997,
using a cell from the udder of a six-year-old sheep. The cloning was achieved by scientists at Roslin
Institute, Scotland, after 276 unsuccessful attempts. By 2006 domestic ferrets were being cloned
successfully with the aim of eventually using them to study diseases such as cystic fibrosis.
With regard to human cloning, past experiments have been few, controversial, and limited in success.
Advanced Cell Technology (ACT), a Massachusetts biotechnology company, cloned human embryos
by inserting a skin cell into enucleated donor eggs in 2001. The chemical ionomycin was used to
incite division. However, of eight eggs, only one divided as far as a six cell stage before the process
ceased. A year later, Clonaid, part of a religious group called the Raelians, announced the birth of the
first cloned human, but has yet to reveal any evidence.
Impacts and Issues
Recombinant DNA Technology
Recombinant DNA technology or molecular cloning has been in use since the 1970s and is
commonplace in the field of molecular biology. In this type of cloning, a fragment of DNA is inserted
into an element capable of self-replication such as a bacterial plasmid, which in turn is inserted into a
host cell, such as bacteria, yeast, or mammalian cells, in order to create multiple copies of the original
gene. The Human GenomeProject, completed in 2003, used molecular cloning to ensure enough
identical genetic material for study and research.
Recombinant DNA technology is a fundamental part of many other related technologies. It is used in genome
sequencing, and also gene therapy, which can be used to correct faulty genes involved in the manifestation of
genetic conditions. In genetic engineering, transgenic crops or genetically-modified organisms (GMOs) can be
created to improve taste, yield, or hardiness. Genetically-modified and cloned microbial cells can also be produced
to digest crude oil spills.
Reproductive Cloning
When cloning is mentioned in the press, it is often in reference to reproductive cloning in animals.
Reproductive cloning is used initially to produce a cell with the same nuclear DNA as its parent cell, a
process also referred to as nuclear transfer technology, with the final goal of producing an entire
cloned organism. After division has been encouraged with either electricity or chemicals, the now
multi-cell embryo is implanted in a surrogate host where it can develop through a normal in utero
developmental period until birth. The clones have the same nuclear DNA as their parent, but the
presence of genetic material in the mitochondria of the donor egg means that the clone is not
completely identical.
Embryonic stem cells in an organism divide and take on specialized roles, eventually becoming the
organs and tissues of the new animal. Scientists for a long time saw this as a permanent one-way
process, with the unused genes turning off, but the experiments with Dolly the sheep at the Roslin
Institute changed this view, because Dolly was cloned using a somatic cell from the udder of another,
older sheep. Even though the nuclear DNA had originally resided in a very specialized cell, the new
organism was able to access the genes for all the requisite differentiation of cells throughout the new
lamb's body.
Reproductive cloning in animals could be used in the future in tandem with genetic engineering to
mass produce transgenic animals with applications in both medicine and farming. Genetically altered
livestock with increased disease resistance or desirable qualities such as leaner meat or higher milk
production could become the norm. The FDA stated in 2008 that any cloned animal products are safe
for human consumption. Transgenic animals already are being used to produce useful products in
their milk, a practice known as ‘pharming.’ Goats are being engineered to produce spider silk, with a
myriad of uses in medicine such as a suture material and artificial tendons. Sheep are producing the
emphysema drug alpha-1 antitrypsin; and rabbits are producing milk with human interleukin.
Reproductive cloning would allow larger scale production of these animals to be possible.
From an environmental standpoint the technology could address the issue of endangered species by
helping to rejuvenate a population, something that has already been tested with the Banteg ox.
Whether cloning would ensure long-term viability of a species is yet to be seen, given the inherent
lack of genetic variability that cloned animals would involve. Recreating extinct animals, a popular
subject of science fiction, raises even more problems. Because the donor egg would come from a
related but not identical species, the mitochondrial DNA contained therein would mean that the
cloned animals could never be exact reproductions of their extinct ancestors.
The success rates for reproductive cloning are low, with only about 10 percent of cloned animals
being viable, and often the statistics are much lower. Primates, for example, are very difficult to clone.
One reason for this involves spindle proteins. These two proteins are essential for correct division of
cells. Whereas other animals have spindle proteins spread throughout the cell, in primates these cells
lay close to the chromosomes, meaning removing the nucleus can damage or destroy them easily.
The adverse effects of cloning on animals are well documented and manifest in a multitude of ways,
from abnormally high birth size to serious defects in organs and a compromised immune system. At
the Whitehead Institute for Biomedical Research in Cambridge, Massachusetts, scientists discovered
that in 4 percent of the genes in cloned animals there was an abnormality. One element is the idea of
cell age. Telomere sequences are regions of repetitive DNA at the ends of chromosomes. Cell
division causes the telomeres to decrease in size, and once they have disappeared, cell senescence
occurs. Clones created from adult cells as in SCNT would already have shortened chromosomes. In
the case of Dolly the sheep, whose genetic material was taken from a six-year-old sheep, she died at
half the expected age for her breed. Dolly also illustrates the effect these aging implications,
programming errors, and abnormalities can have on an organism; she was euthanized by lethal
injection because of lung cancer and severe arthritis.
Human Reproductive Cloning
It is these aforementioned multivariate problems with the reproductive cloning process that have
resulted in an almost worldwide ban against using these techniques in humans. Additionally,
alongside the medical considerations, is the idea of cognition. Humans are higher-order organisms
who rely on intellect and emotional variances to function socially. There is no substantial body of work
addressing how cloning would affect mental functioning. Furthermore, it is considered by many
unethical to proceed with human cloning when animal cloning has produced organisms of such ill
health, especially considering the problems of extracting intact genetic information from primates.
The concept of producing a genetically identical clone is a controversial one. Although it could allow
for sterile couples to have a genetically linked child or could be used with genetic engineering to wipe
out genetic defects in family lines, it also would appear to conflict with the ideas of individuality,
human dignity, and religious principles. Further, the implication that it would allow for the cloning of a
genetically identical lost loved one is a provocative topic.
Full Text: COPYRIGHT 2012 Gale, Cengage Learning.