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The possibility of human cloning,
raised when Scottish scientists at Roslin Institute created the much-celebrated
sheep "Dolly" (Nature 385, 810-13, 1997), has
aroused worldwide interest and concern because of its scientific and ethical
implications.
The feat,
cited by Science magazine as the breakthrough of 1997, also has generated
uncertainty over the meaning of "cloning" --an umbrella
term
traditionally used by scientists to describe different processes for duplicating
biological material.
What is cloning?
To Human Genome Project researchers,
cloning
refers to copying
genes
and other pieces of
chromosomes
to generate enough identical material for further study. Two other types
of cloning produce complete, genetically
identical
animals. Blastomere separation (sometimes called "twinning"
after the naturally occurring process that creates identical twins)
involves
splitting
a developing
embryo
soon after
fertilization
of the egg by a
sperm
(sexual reproduction) to give rise to two or more embryos. The resulting
organisms are identical twins (clones) containing DNA from both the
mother and the father. Dolly, on the other hand, is the result of
another type of cloning that produces an animal carrying the DNA of only
one parent. Using
somatic
cell
nuclear
transfer, scientists transferred genetic material from the nucleus of an
adult sheep's
udder
cell to an egg whose
nucleus,
and thus its genetic material, had been removed. (All cells that are not
egg or sperm cells are somatic cells.)
Why clone?
One goal of this and similar research is to develop efficient ways to
alter animals genetically and reproduce them
reliably.
Alterations
have included adding genes (such as those for human proteins) to create
drug-producing animals as well as
inactivating
genes to study the effects and possibly create animal models of human
diseases. Cloning technology also may someday be used in humans to
produce whole organs from single cells or to raise animals having
genetically altered organs suitable for
transplanting
to humans.
The technique used to produce Dolly and other cloned animals is an
extension of 40 years of research using DNA from nonhuman embryonic and fetal
cells. Before this demonstration, scientists believed that once a cell
became specialized a liver, heart, udder, bone, or any other type of
cell the change was permanent and other unneeded genes in the cell
became inactive. Dolly's creators demonstrated that nuclei
of an adult animal's specialized cells can be made to revert
to a nonspecialized, embryonic state, thus restoring the ability to give
rise to any kind of cell. Explorations into how cells revert to an undifferentiated
state may provide insights into the process by which cells become
cancerous.
Using the same technique that produced Dolly, researchers have cloned
a number of large and small animals including sheep, goats, mice and
cows. But scientists remain uncertain about whether genetic changes in
the cells used to obtain nuclei will lead to adverse
effects on the health of the cloned animals.
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