WHAT IS CLONING?
Have you ever wished you could have a clone of yourself to do homework while you hit the skate
park or went out with your friends? Imagine if you could really do that. Where would you start?
What exactly is cloning?
Cloning is the creation of an organism that is an exact genetic copy of another. This means that
every single bit of DNA is the same between the two! You might not believe it, but there are human clones
among us right now. They weren't made in a lab, though: they're identical twins, created naturally. Below,
we'll see how natural identical twins relate to modern cloning technologies.
How is cloning done?
You may have first heard of cloning when Dolly the Sheep showed up on the scene in 1997.
Cloning technologies have been around for much longer than Dolly, though. 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?
1. Artificial Embryo Twinning
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. Each cell continues dividing on its own,
ultimately developing into a separate individual within the mother. Since the two cells came from the same
zygote, the resulting individuals are genetically identical. Artificial embryo twinning uses the same
approach, but it occurs in a Petri dish instead of in the mother's body. 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.
What does SCNT mean? Let's take it apart:
Somatic cell: A somatic cell is any cell in the body other than the two types of reproductive
cells, sperm and egg. These are also called germ cells. In mammals, every somatic cell
has two complete sets of chromosomes, whereas the germ cells only have one complete
Nuclear: The nucleus is like the cell's brain. It's an enclosed compartment that contains all
the information that cells need to form an organism. This information comes in the form of
DNA. It's the differences in our DNA that make each of us unique.
Transfer: Moving an object from one place to another.
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 tweaks, 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 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?
The fertilization of an egg by a sperm and the SCNT cloning method both result in the same thing:
a dividing ball of cells, called an embryo. So what exactly is the difference between these methods? An
embryo is composed of cells that contain 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.
Research advances over the past decade have told us that, with a little work, we humans can
clone just about anything we want, from frogs to monkeys and probably even ourselves! So, we can clone
things, but why would we want to? Let's look at some of the reasons people give to justify cloning.
1. Cloning for medical purposes
Of all the reasons, cloning for medical purposes has the most potential to benefit large
numbers of people. How might cloning be used in medicine?
Cloning animal models of disease
Much of what researchers learn about human disease comes from studying animal models such as
mice. Often, animal models are genetically engineered to carry disease-causing mutations in their
genes. Creating these transgenic animals is a time-intensive process that requires trial-and-error
and several generations of breeding. Cloning technologies might reduce the time needed to make
a transgenic animal model, and the result would be a population of genetically identical animals for
Cloning stem cells for research
Stem cells are the body's building blocks, responsible for developing, maintaining and repairing the
body throughout life. As a result, they might be used to repair damaged or diseased organs and
tissues. Researchers are currently looking toward cloning as a way to create genetically defined
human stem cells for research and medical purposes. To see how this is done, see Creating Stem
Cells for Research, a component of the Stem Cells in the Spotlight module.
"Pharming" for drug production
Farm animals such as cows, sheep and goats are currently being genetically engineered to
produce drugs or proteins that are useful in medicine. Just like creating animal models of disease,
cloning might be a faster way to produce large herds of genetically engineered animals. Find out
more about this research in the feature article Pharming for Farmaceuticals.
2. Reviving Endangered or Extinct Species
Have you seen Jurassic Park? In this feature film, scientists use DNA preserved for tens
of millions of years to clone dinosaurs. They find trouble, however, when they realize that
the cloned creatures are smarter and fiercer than expected. Could we really clone
dinosaurs? In theory? Yes. What would you need to do this?
A well-preserved source of DNA from the extinct dinosaur, and
A closely related species, currently living, that could serve as a surrogate mother
In reality? Probably not. It's not likely that dinosaur DNA could survive undamaged for such a long time.
However, scientists have tried to clone species that became extinct more recently, using DNA from well-
preserved tissue samples. For an example, see "Can we really clone endangered or extinct animals?".
3. Reproducing a Deceased Pet
No joke! If you really wanted to, and if you had enough money, you could clone your
beloved family cat. At least one biotechnology company in the United States offers cat cloning
services for the privileged and bereaved, and they are now working to clone dogs. But don't
assume that your cloned kitty will be exactly the same as the one you know and love. Why not?
See Cloning Myths.
4. Cloning Humans?
To clone or not to clone: that is the question. The prospect of cloning humans is highly
controversial and raises a number of ethical, legal and social challenges that need to be
considered. To explore some of these, see What are Some Issues in Cloning? Why would
anyone want to clone humans? Some reasons include:
To help infertile couples have children
To replace a deceased child
In What is cloning? we learned what it means to clone an individual organism. Given its high profile
in the popular media, the topic of cloning brings up some common, and often confusing, misconceptions.
Misconception #1: Instant Clones!
Let's say you really wanted a clone to do your homework. After
reviewing What is Cloning? and Click and Clone, you've figured out,
generally, how this would be done. Knowing what you know, do you
think this approach would really help you finish your homework...this
A common misconception is that a clone, if created, would
magically appear at the same age as the original. This simply isn't
true. You remember that cloning is an alternative way to create an
embryo, not a full-grown individual. Therefore, that embryo, once
created, must develop exactly the same way as would an embryo
created by fertilizing an egg cell with a sperm cell. This will require a
surrogate mother and ample time for the cloned embryo to grow and
fully develop into an individual.
Misconception #2: Carbon Copies!
Your beloved cat Frank has been a loyal companion for
years. Recently, though, Frank is showing signs of old age, and you realize that your friend's days are
numbered. You can't bear the thought of living without her, so you contact a biotechnology company that
advertises pet cloning services. For a fee, this company will clone Frank using DNA from a sample of her
somatic cells. You're thrilled: you'll soon have a carbon copy of Frank - we'll call her Frank #2 - and you'll
never have to live without your pal! Right?
Not exactly. Are you familiar with the phrase "nature versus nurture?" Basically, this means that
while genetics can help determine traits, environmental influences have a considerable impact on shaping
an individual's physical appearance and personality. For example, do you know any identical twins? They
are genetically the same, but do they really look and act exactly alike?
So, even though Frank #2 is genetically identical to the original Frank, she will grow and develop in
a completely different environment than the original Frank or will have a different mother, and she will be
exposed to different experiences throughout her development and life. Therefore, there is only a slim
chance that Frank #2 will closely resemble the Frank you know and love.
Here, kitty, kitty
On December 22, 2001, a kitten named CC made history as
the first cat - and the first domestic pet - ever to be cloned. CC and
Rainbow, the donor of CC's genetic material, are pictured.
But do you notice something odd about this picture? If CC is a clone
- an exact genetic copy - of Rainbow, then why don't they look
The answer lies on the X chromosome. In cats, a gene that
helps determine coat color resides on this chromosome. Both CC
and Rainbow, being females, have two X chromosomes. (Males
have one X and one Y chromosome.) Since the two cats have the exact same X chromosomes, they have
the same two coat color genes, one specifying black and the other specifying orange.
So why do they look different? Very early in her development, each of Rainbow's cells "turned off"
one entire X chromosome - and therefore, turned off either the black color gene or the orange one. This
process, called X-inactivation, happens normally in females, in order to prevent them from having twice as
much X-chromosome activity as males. It also happens randomly, meaning that not every cell turns off the
same X chromosome.
As a result, Rainbow developed as a mosaic of cells that had one or the other coat color gene
inactivated - some patches of cells specified black, other patches specified orange, and still others
specified white, due to more complex genetic events. This is how all calico cats, like Rainbow, get their
CC looks different because the somatic cell that Rainbow donated to create her contained an
activated black gene and an inactivated orange gene. What's interesting is that, as CC developed, her cells
did not change that inactivation pattern. Therefore, unlike Rainbow, CC developed without any cells that
specified orange coat color. The result is CC's black and white tiger-tabby coat. Rainbow and CC are living
proof that a clone will not look exactly like the donor of its genetic material.
WHAT ARE THE RISKS OF CLONING?
When we hear of cloning successes, we learn about only the few attempts that worked. What we don't see
are the many, many cloning experiments that failed! And even in the successful clones, problems tend to
arise later, during the animal's development to adulthood. Cloning animals shows us what might happen if
we try to clone humans. What have these animals taught us about the risks of cloning?
1. High failure rate
Cloning animals through somatic cell nuclear transfer is simply inefficient. The success rate ranges
from 0.1 percent to 3 percent, which means that for every 1000 tries, only one to 30 clones are made. Or
you can look at it as 970 to 999 failures in 1000 tries. That's a lot of effort with only a speck of a return!
Why is this? Here are some reasons:
The enucleated egg and the transferred nucleus may not be compatible
An egg with a newly transferred nucleus may not begin to divide or develop properly
Implantation of the embryo into the surrogate mother might fail
The pregnancy itself might fail
2. Problems during later development
Cloned animals that do survive tend to be much bigger at birth than their natural counterparts. Scientists
call this "Large Offspring Syndrome" (LOS). Clones with LOS have abnormally large organs. This can lead
to breathing, blood flow and other problems. Because LOS doesn't always occur, scientists cannot reliably
predict whether it will happen in any given clone. Also, some clones without
LOS have developed kidney or brain malformations and impaired immune
systems, which can cause problems later in life.
3. Abnormal gene expression patterns
Are the surviving clones really clones? The clones look like the
originals, and their DNA sequences are identical. But will the clone express
the right genes at the right time? In Click and Clone, we saw that one
challenge is to re-program the transferred nucleus to behave as though it
belongs in a very early embryonic cell. This mimics natural development,
which starts when a sperm fertilizes an egg. In a naturally-created embryo,
the DNA is programmed to express a certain set of genes. Later on, as the
embryonic cells begin to differentiate, the program changes. For every type of
differentiated cell - skin, blood, bone or nerve, for example - this program is
different. In cloning, the transferred nucleus doesn't have the same program
as a natural embryo. It is up to the scientist to reprogram the nucleus, like
teaching an old dog new tricks. Complete reprogramming is needed for
normal or near-normal development. Incomplete programming will cause the
embryo to develop abnormally or fail.
4. Telomeric differences
As cells divide, their chromosomes get shorter. This is because the
DNA sequences at both ends of a chromosome, called telomeres, shrink in
length every time the DNA is copied. The older the animal is, the shorter its
telomeres will be, because the cells have divided many, many times. This is a
natural part of aging. So, what happens to the clone if its transferred nucleus
is already pretty old? Will the shortened telomeres affect its development or
lifespan? When scientists looked at the telomere lengths of cloned animals,
they found no clear answers. Chromosomes from cloned cattle or mice had
longer telomeres than normal. These cells showed other signs of youth and
seemed to have an extended lifespan compared with cells from a naturally
conceived cow. On the other hand, Dolly the sheep's chromosomes had
shorter telomere lengths than normal. This means that Dolly's cells were aging
faster than the cells from a normal sheep. To date, scientists aren't sure why
cloned animals show differences in telomere length.
WHAT ARE SOME ISSUES IN CLONING?
We saw in What are the Risks of Cloning? that the success rate in
cloning is quite low. Even if we can increase the odds of success, problems
can arise during the clone's development, both before and after pregnancy.
Despite these risks, supporters of human reproductive cloning see it as a
possible solution to infertility problems. Others support therapeutic cloning to
create embryonic stem cells for research and medicine.
What are the possible implications of cloning to society? All of us -
researchers, policymakers and the public - have a responsibility to explore the
potential effects of cloning technologies on our lives so that we can make
informed decisions. For each new application of cloning technologies, we
What are the benefits?
What are the risks?
Whom will the technology help? Does it have the potential to hurt anyone?
What does this mean for me? For my family? For others around me?
Why might others not share my view?
Ethical, legal and social issues.
There are several types of issues to consider as we think about cloning.
Ethical issues are those that ask us to consider the potential moral outcomes of cloning
Legal issues require researchers and the public to help policymakers decide whether and how
cloning technologies should be regulated by the government.
Social issues involve the impact of cloning technologies on society as a whole.
Some questions to ponder.
The questions raised here have no clear right or wrong answer. Instead, your response will depend on your
own set of values, as well as the opinions of those around you.
Who has the right to have children, no matter how they are created? Who doesn't? Why?
Is human cloning "playing with nature?" If so, how does that compare with other reproductive
technologies such as in vitro fertilization or hormone treatments?
Does cloning to create stem cells, also called therapeutic cloning, justify destroying a human
embryo? Why, or why not?
If a clone originates from an existing person, who is the parent?
What are some of the social challenges a cloned child might face?
Do the benefits of human cloning outweigh the costs of human dignity?
Should cloning research be regulated? How, and by whom?