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What is cloning? Are there different types of cloning?
When the media report on cloning in the news, they are usually
talking about only one type of cloning called reproductive
cloning. It is extremely important to know that there are
different types of cloning and that cloning technologies can
be used for other purposes besides producing the genetic twin
of another organism. A basic understanding of the different
types of cloning is key to taking a more informed stance on
current public policy issues and for making the best possible
personal decisions. The following three types of cloning technologies
will be discussed: (1) recombinant DNA technology or DNA cloning,
(2) reproductive cloning, and (3) therapeutic cloning.
Recombinant DNA Technology or DNA Cloning
The terms "recombinant DNA technology," "DNA
cloning," "molecular cloning,"or "gene
cloning" all refer to the same process: the transfer
of a DNA fragment of interest from one organism to a self-replicating
genetic element such as a bacterial plasmid. The DNA of interest
can then be propagated in a foreign host cell. This technology
has been around since the 1970's, and it has become a common
practice in molecular biology labs today.
Scientists studying a particular gene often use bacterial
plasmids to generate multiple copies of the same gene. Plasmids
are self-replicating extra-chromosomal circular DNA molecules,
distinct from the normal bacterial genome. Plasmids and other
types of cloning vectors are used by Human Genome Project
researchers to copy genes and other pieces of chromosomes
to generate enough identical material for further study.
To "clone a gene," a DNA fragment containing the
gene of interest is isolated from chromosomal DNA using restriction
enzymes and then united with a plasmid that has been cut with
the same restriction enzymes. When the fragment of chromosomal
DNA is joined with its cloning vector in the lab, it is called
a "recombinant DNA molecule." Following introduction
into suitable host cells, the recombinant DNA can then be
reproduced along with the host cell DNA.
Plasmids can carry up to 20,000 bp of foreign DNA. Besides
bacterial plasmids, some other cloning vectors include viruses,
cosmids (artificially constructed cloning vectors that carry
up to 45 kb of foreign DNA and can be packaged in lambda phage
particles for infection into E. coli cells), bacteria artificial
chromosomes called BACs (utilize the naturally occurring F-factor
plasmid found in E. coli to carry 100-300 kb DNA inserts)
and yeast artificial chromosomes, which can carry up to 1
MB of foreign DNA. Bacteria are most often used as the host
cells for recombinant DNA molecules, but yeast and mammalian
cells are also used.
Reproductive Cloning
Reproductive cloning is a technology used to generate an
animal that has the same nuclear DNA as another currently
or previously existing animal. Dolly 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. Acquired
mutations in mitochondrial DNA are believed to play an important
role in the aging process.
Dolly's success is truly remarkable because it proved that
the genetic material from a specialized adult cell, such
as an udder cell programmed to express only those genes needed
by udder cells, could be reprogrammed to generate an entire
new organism. Before this demonstration, scientists believed
that once a cell became specialized as a liver, heart, udder,
bone, or any other type of cell, the change was permanent
and other unneeded genes in the cell would become inactive.
Some scientists believe that errors or incompleteness in
the reprogramming process cause the high rates of death,
deformity, and disability observed among animal clones.
Therapeutic Cloning
Therapeutic cloning, also called "embryo cloning," is the
production of human embryos for use in research. The goal
of this process is not to create cloned human beings, but
rather to harvest stem cells that can be used to study human
development and to treat disease. Stem cells are important
to biomedical researchers because they can be used to generate
virtually any type of specialized cell in the human body.
Stem cells are extracted from the egg after it has divided
for 5 days. The egg at this stage of development is called
a blastocyst. The extraction process destroys the embryo,
which raises a variety of ethical concerns. Many researchers
hope that one day stem cells can be used to serve as replacement
cells to treat heart disease, Alzheimer's, cancer, and other
diseases. See more on the potential
use of cloning in organ transplants.
In November 2001, scientists from Advanced Cell Technologies
(ACT), a biotechnology company in Massachusetts, announced
that they had cloned the first human embryos for the purpose
of advancing therapeutic research. To do this, they collected
eggs from women's ovaries and then removed the genetic material
from these eggs with a needle less than 2/10,000th of an
inch wide. A skin cell was inserted inside the enucleated
egg to serve as a new nucleus. The egg began to divide after
it was stimulated with a chemical called ionomycin. The results
were limited in success. Although this process was carried
out with eight eggs, only three began dividing, and only
one was able to divide into six cells before stopping.
To read the rest
of this excellent article on cloning, produced by the Human
Genome Project, please click here: http://www.ornl.gov/TechResources/Human_Genome/elsi/cloning.html
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