Imagine having
a copy of yourself, similar to having an identical twin but an exact replica
of you that is created from your own cells. Such a dream opened the door for
cloning.
WHAT IS CLONING
In general; it
is the replication of certain type of cells from a "parent" cell or the
replication of a certain part of the cell/ DNA to propagate a certain
desirable genetic trait.
To understand this subject we have to differentiate between its different types:
1. DNA cloning
2. Therapeutic cloning
3. Reproductive cloning
DNA CLONING
This means the transfer of a A fragment of interest from one organism to a self
replicating genetic element such as bacterial plasmid so DNA of interest can
then be propagated in foreign host cells.
THERAPEUTIC CLONING
Also called "embryo cloning". It is production of human embryos for use in search
i.e. taking stem cells from the produced embryos for study & for producing tissues
to treat diseases.
Reproductive cloning:
A technology used to generate an animal that has the same nuclear DNA as another
currently or previously existing one. i.e. every single gene is exactly the
same between the two in a process called "Somatic Cell Nuclear Transfer" or (SCNT) and
this type of cloning is our subject to discuss.
HISTORICAL VIEW
Scientists have been cloning animals for many years. In 1952, the first animal,
a tadpole, was cloned. Before the creation of Dolly the sheep at 1997 by a Scottish
scientist at Rosaline institute, the first mammal cloned from the cell of an
adult animal, clones were created from embryonic cells. Since Dolly, researchers
have cloned a number of large and small animals including sheep, goats, cows,
mice, pigs, cats, rabbits, and a gaur this give raise of possibility of human
cloning.
How is cloning done?.
How does one go about making an exact genetic copy of an organism? There are
a couple of ways to do this: artificial embryo twinning and somatic cell nuclear
transfer. How do these processes differ?
Artificial Embryo Twinning
1. Artificial embryo twinning is the relatively low-tech version of cloning.
As the name suggests, this technology mimics the natural process of creating
identical twins.
In nature, twins occur just after fertilization of an egg cell by a sperm cell.
In rare cases, when the resulting fertilized egg, called a zygote, tries to
divide into a two-celled embryo, the two cells separate completely. Each cell
mass continues
dividing on its own, ultimately developing into a separate individual within
the mother. Since the two cell masses came from the same zygote, the resulting individuals
are genetically identical twins.
Artificial embryo twinning uses the same approach, but it occurs in a Petri
dish instead of the mother's uterus. This is accomplished by manually separating
a very early embryo into individual cells, and then allowing each cell to divide
and develop on its own. The resulting embryos are placed into a surrogate mother,
where they are carried to term and delivered. Again, since all the embryos came
from the same zygote, they are genetically identical.
2. Somatic Cell Nuclear Transfer
Somatic cell nuclear transfer (SCNT) uses a different approach than artificial
embryo twinning, but it produces the same result: an exact clone, or genetic
copy, of an individual. This was the method used to create Dolly the Sheep.
To make Dolly, researchers isolated a somatic cell from an adult female sheep.
Next, they transferred the nucleus from that cell to an egg- cell from which
the nucleus had been removed. After a couple of chemical
reactions/interventions, the egg cell,
with its new nucleus, was behaving just like a freshly fertilized zygote. It
developed into an embryo, which was implanted into a surrogate mother and carried
to term.
The lamb, Dolly, was an exact genetic replica of the adult female sheep that
donated the somatic cell-nucleus to the enucleated egg. She was the first-ever mammal to
be cloned from an adult somatic cell.
How does SCNT differ from the natural way of making an embryo?
An embryo is composed of cells containing two complete sets of chromosomes.
The difference between fertilization and SCNT lies in where those two sets originated.
In fertilization, the sperm and egg both contain one set of chromosomes. When
the sperm and egg join, the resulting zygote ends up with two sets; one from
the father (sperm) and one from the mother (egg).
In SCNT, the egg cell's single set of chromosomes is removed. It is replaced
by the nucleus from a somatic cell, which already contains two complete sets
of chromosomes. Therefore, in the resulting embryo, both sets of chromosomes
come from the somatic cell.
What are the risks of cloning ?
Reproductive cloning is expensive and highly inefficient. More than 90% of cloning
attempts fail to produce a viable offspring. More than 100 nuclear transfer procedures
could be required to produce one viable clone. In addition to low success rates,
cloned animals tend to have more compromised immune function and higher rates
of infection, tumor growth, and other disorders. Japanese studies have shown
that cloned mice live in poor health and die early. About a third of the cloned
calves born alive have died young, and many of them were abnormally large. Many
cloned animals have not lived long enough to generate good data about how clones
age. Appearing healthy at a young age unfortunately is not a good indicator
of long term survival. Clones have been known to die mysteriously. For example,
Australia's first cloned sheep appeared healthy and energetic on the day she
died, and the results of her autopsy failed to determine a cause of death.
Should humans be cloned?
Due to the inefficiency of animal cloning (only about 1 or 2 viable offspring
for every 100 experiments) and the lack of understanding about reproductive
cloning, many scientists and physicians strongly believe that it would be unethical
to attempt to clone humans. Not only do most attempts to clone mammals fail,
about 30% of clones born alive are affected by "large offspring syndrome"
and other debilitating conditions. Several cloned animals have died prematurely
from infections and other complications. The same problems would be expected
in human cloning. In addition, scientists do not know how cloning could impact
mental development. While factors such as intellect and mood may not be as important
for a cow or a mouse, they are crucial for the development of healthy humans.
With so many unknowns concerning reproductive cloning, the attempt to clone
humans at this time is considered potentially dangerous and ethically irresponsible.
Embryos might result from syngamy between a gamete nucleus and a haploidized
somatic cell nucleus and the somatic cell may replace either the male or
the female gamete, which makes this technique potentially applicable in the
treatment of both male and female infertility. The validity of haploidization
is controversial and will require further research. Though sharing some features
with conventional cloning, the technique is biologically closer to normal fertilization
because the future individual results from the union of two parental genomes
of which one is actually brought by a gamete. This would alleviate some of the
ethical concerns raised against the use of conventional cloning in human assisted
reproduction.
