Thames & Kosmos Genetics & DNA handleiding

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Thames & Kosmos

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Handleiding

DOLLY

How humans tricked the rules of heredity

Dolly was a Scottish sheep born on July 5, 1996. Just

under a year later, as the world learned about her,

she caused a worldwide commotion. With this

famous sheep, researchers were able for the ﬁrst

time to clone, or copy, a mammal. In nature, that

isn’t too unusual. Plant cuttings, for example, are

clones, and the plants are genetically identical.

People have long been able to clone frogs. But with

mammals, clones are a rare event: identical twins

are genetically identical, and they really do look so

similar that it’s hard to tell them apart. They come

about when the egg cell divides into two

independent embryos following fertilization.

But the idea of creating a genetic copy of a

mammal — such as a cow, dog, sheep, goat, cat, or

even a human — remained in the realm of science

ﬁction until Dolly came along. This is the trick the

researchers used: They removed the cell nucleus

from a normal somatic (body) cell of the animal. At

the same time, they take the cell nucleus from the

egg cell of another animal and inject the ﬁrst

nucleus into it. Then, they trick the egg cell into

thinking that it has been fertilized, and it starts to

divide. The researchers then transfer the artiﬁcial

embryo into the womb of another animal, which in

the ideal case will end up bringing an intact animal

into the world.

Cloning has since been performed on dogs, cats,

mice, horses, and rats. But the success rate is very

low. Only a few embryos survive, and quite a few

that do are born with defects — a high price to pay

for the realization of an idea.

Nobody has ever succeeded in cloning a human,

and there are many moral and ethical issues

involved in such a pursuit.

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Cells and Chromosomes | 21

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