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How Cloning Works
by Craig C. Freudenrich, Ph.D.
On January 8, 2001, scientists at Advanced Cell Technology, Inc.,
announced the birth of the first clone of an endangered animal, a
baby bull gaur (a large wild ox from India and southeast Asia)
named Noah. Although Noah died of an infection unrelated to the
procedure, the experiment demonstrated that it is possible to save
endangered species through cloning.
Cloning is the process of making a genetically identical organism
through nonsexual means. It has been used for many years to
produce plants (even growing a plant from a cutting is a type of
cloning). Animal cloning has been the subject of scientific
experiments for years, but garnered little attention until the birth of
the first cloned mammal in 1997, a sheep named Dolly. Since
Dolly, several scientists have cloned other animals, including cows
and mice. The recent success in cloning animals has sparked
fierce debates among scientists, politicians and the general public
about the use and morality of cloning plants, animals and possibly
humans.
Photo courtesy Advanced Cell Technology,
Inc.
Noah was the first endangered
animal to be cloned.
In this article, we will examine how cloning works and look at possible uses of this technology.
http://science.howstuffworks.com/cloning.htm
Producing Clones: Plant Life
Nature has been cloning organisms for billions of years. For example, when a
strawberry plant sends out a runner (a form of modified stem), a new plant grows
where the runner takes root. That new plant is a clone. Similar cloning occurs in
grass, potatoes and onions.
People have been cloning plants in one way or another for thousands of years.
For example, when you take a leaf cutting from a plant and grow it into a new
plant (vegetative propagation), you are cloning the original plant because the
new plant has the same genetic makeup as the donor plant. Vegetative
propagation works because the end of the cutting forms a mass of nonspecialized cells called a callus. With luck, the callus will grow, divide and form
various specialized cells (roots, stems), eventually forming a new plant.
Reproduction
Sexual reproduction involves
the merging of two sets of DNA
(one from the father's sperm
and one from the mother's egg)
to produce a new offspring that
is genetically different from
either parent. Asexual
reproduction (without sex)
produces offspring that are
genetically identical to the single
parent organism.
More recently, scientists have been able to clone plants by taking pieces of specialized roots, breaking them
up into root cells and growing the root cells in a nutrient-rich culture. In culture, the specialized cells become
unspecialized (dedifferentiated) into calluses. The calluses can then be stimulated with the appropriate plant
hormones to grow into new plants that are identical to the original plant from which the root pieces were
taken.
Tomado de http://science.howstuffworks.com
Adaptado por: Elba M. Sepúlveda para AlACiMa
verano 2005
Diagram of plant cloning through tissue culture propagation
This procedure, called tissue culture propagation, has been widely used by horticulturists to grow prized
orchids and other rare flowers.
http://science.howstuffworks.com/cloning1.htm
Producing Clones: Animal Kingdom
Plants are not the only organisms that can be cloned naturally. The unfertilized eggs of some animals (small
invertebrates, worms, some species of fish, lizards and frogs) can develop into full-grown adults under certain
environmental conditions -- usually a chemical stimulus of some kind. This process is called
parthenogenesis, and the offspring are clones of the females that laid the eggs.
Another example of natural cloning is identical twins. Although they are genetically different from their
parents, identical twins are naturally occurring clones of each other.
Scientists have experimented with animal cloning, but have never been able to stimulate a specialized
(differentiated) cell to produce a new organism directly. Instead, they rely on transplanting the genetic
information from a specialized cell into an unfertilized egg cell whose genetic information has been destroyed
or physically removed.
In the 1970s, a scientist named John Gurdon successfully cloned tadpoles. He transplanted the nucleus from
a specialized cell of one frog (B) into an unfertilized egg of another frog (A) in which the nucleus had been
destroyed by ultraviolet light. The egg with the transplanted nucleus developed into a tadpole that was
genetically identical to frog B.
Tomado de http://science.howstuffworks.com
Adaptado por: Elba M. Sepúlveda para AlACiMa
verano 2005
Gurdon's experiment to clone a frog
While Gurdon's tadpoles did not survive to grow into adult frogs, his experiment showed that the process of
specialization in animal cells was reversible, and his technique of nuclear transfer paved the way for later
cloning successes.
http://science.howstuffworks.com/cloning2.htm
Dolly
In 1997, cloning was revolutionized when Ian Wilmut and his colleagues at the Roslin Institute in Edinburgh,
Scotland, successfully cloned a sheep named Dolly. Dolly was the first cloned mammal.
Photo courtesy Roslin Institute
Dolly (left)
Wilmut and his colleagues transplanted a nucleus from a mammary gland cell of a Finn Dorsett sheep into the
enucleated egg of a Scottish blackface ewe. The nucleus-egg combination was stimulated with electricity to
fuse the two and to stimulate cell division. The new cell divided and was placed in the uterus of a blackface
ewe to develop. Dolly was born months later.
Dolly was shown to be genetically identical to the Finn Dorsett mammary cells and not to the blackface ewe,
which clearly demonstrated that she was a successful clone (it took 276 attempts before the experiment was
successful). Dolly has since grown and reproduced several offspring of her own through normal sexual
means. Therefore, Dolly is a viable, healthy clone.
Tomado de http://science.howstuffworks.com
Adaptado por: Elba M. Sepúlveda para AlACiMa
verano 2005
Diagram of the nuclear transfer procedure that produced the
first cloned mammals
Since Dolly, several university laboratories and companies have used various modifications of the nuclear
transfer technique to produce cloned mammals, including cows, pigs, monkeys, mice and Noah.
Why Clone?
The main reason to clone plants or animals is to mass produce organisms with desired qualities, such as a
prize-winning orchid or a genetically engineered animal -- for instance, sheep have been engineered to
produce human insulin. If you had to rely on sexual reproduction (breeding) alone to mass produce these
animals, then you would run the risk of breeding out the desired traits because sexual reproduction reshuffles
the genetic deck of cards.
Other reasons for cloning might include replacing lost or deceased family pets and repopulating endangered
or even extinct species. Whatever the reasons, the new cloning technologies have sparked many ethical
debates among scientists, politicians and the general public. Several governments have considered or
enacted legislation to slow down, limit or ban cloning experiments outright. It is clear that cloning will be a part
of our lives in the future, but the course of this technology has yet to be determined.
http://science.howstuffworks.com/cloning4.htm
Tomado de http://science.howstuffworks.com
Adaptado por: Elba M. Sepúlveda para AlACiMa
verano 2005