HUMAN BIOLOGY

DNa, GeNes, aND BioteChNoloGy 421

to date many clones have developed serious health

problems and have aged much more rapidly than usual.

this happens when the substituted adult nucleus does

not revert to an embryonic state (when a cell is not yet

differentiated for its final function

in the body). Getting a nucleus to

“reprogram” in this way is tricky and

often is not successful.

even so, efforts are under way

in many laboratories to perfect

methods for making cloned embryos

as well as healthy adult animal clones.

such embryos might be used for

therapeutic cloning—that is, as a source of embryonic

stem cells that can be used to grow replacement human

tissues and organs. some companies have announced

plans for reproductive cloning—creating a cloned

embryo that can be implanted in a woman’s uterus and

allowed to develop into a baby. so-called germline

engineering would take the technology a step further by

altering the cloned embryo’s genetic makeup in some

desired way before implanting it.

We humans have been doing genetic experiments for

centuries by manipulating matings to create modern crop

plants and new breeds of animals. Going forward, the

real issue will be how to bring about beneficial changes

without doing harm.

Many people believe that genetic technologies are a double-
edged sword: they can be used for good or ill. some
say that no matter what the species of organism, DNa
should never be altered—even though natural mutations
change DNa all the time. Cloning, or making a genetic
copy of a cell or organism, has become another issue.
Geneticists have become increasingly expert in applying this
technology, from the cloned bacteria used in recombinant
DNa technology to embryos cloned to obtain stem cells
and cloned adult animals.

Cloning of bacteria, plants, and nonhuman
animals raises concerns

there are pros and cons associated with many current
uses of biotechnology. For instance, most recombinant
bacteria or viruses are altered in ways that will prevent them
from reproducing outside a laboratory. in theory, though,
transgenic bacteria or viruses could mutate and possibly
become new pathogens in the process. on the other hand,
genetically engineered “oil-eating” bacteria have been used
to help clean up oil spills, an example of bioremediation.
the bioengineering of plants has both critics and
supporters. Critics point out that such plants could escape
from test plots and become “superweeds” that are resistant
to herbicides and other controls. it is also possible that crop
plants with engineered insect resistance could trigger the
evolution of new, even worse pests. although experts deem
many, even most, of these possibilities to be unlikely, there
are documented cases of engineered plant genes turning
up in wild plants.

Controversy swirls over cloning

scientists have achieved considerable success in cloning
embryos and whole animals of several species. the 1997
cloning of a fully grown sheep named Dolly has been followed
by a string of similar experiments that have produced clones
of cattle, rabbits, pigs, mules, goats, and other animals,
including horses (Figure 21.23). Uses for adult animal clones
range from saving endangered species from extinction
to cloning livestock with desirable traits to “replacing” a
beloved pet that has died.
to make a clone of an adult animal, the nucleus of an
unfertilized egg cell is removed and replaced with the
nucleus from some type of adult cell. an embryo then may
develop from the cell. in theory, the embryo can become
a source of stem cells (Chapter 4), or it may grow into an
adult. either way, the clone contains only the DNa of the
original adult cell.

21.12 to Clone or not to Clone?

SCIENCE COMES TO LIFE

reproductive cloning

Creating a cloned embryo to

produce a pregnancy.

therapeutic cloning

Creating a cloned embryo

that will be used as a source

of embryonic stem cells.

Figure 21.23 Cloned horses and other animals are a

modern-day reality. Prometea, the horse in the foreground,

developed from an embryo produced by cloning, using a skin

cell from the mare in the background. With time, Prometea’s

mane color and other features will match those of her identical

twin. (Mauro Fermariello/Science Source)

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