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
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,
Noah was the first
endangered animal to be
In this edition of HowStuffWorks,
we will examine how cloning works and look at possible uses of
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.
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 non-specialized cells called a callus.
With luck, the callus will grow, divide and form various
specialized cells (roots,
stems), eventually forming a new plant.
ReproductionSexual 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
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.
Diagram of plant cloning through tissue
This procedure, called tissue culture propagation,
has been widely used by horticulturists to grow prized orchids
and other rare flowers.
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
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.
Gurdon's experiment to clone a
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
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
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
Diagram of the nuclear transfer procedure
that produced the first cloned
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,
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.
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.
For more information, check out the links on the next page.
Lots More Information!
More Great Links
Ethics of Cloning