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568 Chapter 17 NEL

Cloning from adult mammalian cells has proved even more difficult, since they tend

to be highly specialized. Until recently, the only way to get clones was by splitting off cells

from a developing embryo (Figure 6). However, cells beyond the eight-cell stage of devel-

opment seem to be unable to stimulate cell division.

donor mouse (white)

Nucleus from

donor is injected

into enucleated

egg.

Unfertilized egg

is removed and

enucleated.

recipient mouse (brown)

Cell is cultured

in laboratory.

Cell mass is

implanted

in recipient.

cloned offspring

(white)

Developing cells from early

stage of embryo

development are collected.

Single cells

are isolated

and nucleus

is extracted.

Figure 6

Cloning a mammal using embryo splitting

The long-held scientific belief that adult cells cannot be used to clone animals was

disproved with the appearance of a sheep named Dolly. Dr. Ian Wilmut, of the Rosalind

Institute in Scotland, extracted the nucleus from an udder cell of an adult Finn Dorsett

sheep and placed the nucleus into the enucleated egg cell from a Poll Dorsett sheep. The

egg was allowed to develop in a Petri dish until an early embryo stage was reached.

Then this embryo was placed into the womb of a third sheep, a Scottish Blackface. Her

genetic information was shown to be identical to that of the Finn Dorsett adult; Dolly

was a clone (Figure 7).

Medical experimentation and research could potentially benefit from the availability

of cloned animals. For example, experiments on the effectiveness of a drug are often

difficult to interpret because of the genetic variation among the individuals tested. If all

the test subjects were genetically identical, clearer results could be obtained. In agricul-

ture, the strongest livestock could be cloned, decreasing farmers’ losses due to disease, and

thereby increasing yield. However, many people have moral and ethical problems with

this technology and worry about the impact on society.

Figure 7
Dolly could claim three different
sheep as mothers. The genetic
mother died before Dolly was born.