Biology 12

Chapter 9 DNA Mutations and Genetic Engineering • MHR 303

9.3 The Chimera: From Legend to Lab

The chimera is described in Greek mythology as a
fire-breathing monster with the head and shoulders
of a lion, the body of a goat, and a serpent for a tail
(see Figure 9.20). Today, geneticists often use the
term “chimera” to describe genetically engineered
organisms that contain genes from unrelated species.
The name may prove very fitting. The mythical
chimera combined the strengths of many different
animals to produce one creature that was
exceptionally powerful. But this chimera was also
frightening — it breathed fire and could be ferocious.
In the same way, modern genetic chimeras bring
together elements of different genomes in ways that
can produce important social benefits. These new
chimeras and the genetic technologies that create
them also pose some disturbing risks; consequently,
many people consider them to be dangerous.

Figure 9.20The chimera is a mythical beast that combines
parts of a lion, goat, serpent, and dragon.

As you saw in the last section, the first chimeric
organism was created in 1973 when Stanley Cohen
and Herbert Boyer successfully developed a bacterial
plasmid that could express an amphibian gene.
The work initiated by Cohen and Boyer remains
the foundation of much of the genetic engineering
done today.

Recombinant DNA Technology

All mammals produce a growth hormone called

somatotropin. When cows are treated with high

levels of this hormone they grow bigger, develop

larger udders, and produce more milk than they

normally would. In 1990, the gene coding for this

hormone in cattle (bovine somatotropin, or BST)

was successfully cloned and inserted into a

bacterial vector using recombinant DNA technology.

Produced on a commercial scale, the resulting

hormone became the first transgenic, or genetically

engineered, product approved for agricultural

use in North America.

To insert a mammalian gene into a prokaryotic

cell, two basic requirements must be met. First,

researchers must isolate the target mammalian gene

from the genome as a whole. Second, the researchers

must find a way to ensure that the prokaryotic cell

can express the mammalian gene correctly.

Creating and Isolating the Target Gene

In the previous section, you learned how

restriction endonucleases can be used to break

DNA into fragments, and how these fragments can

then be inserted into bacterial plasmids for cloning.

Recombinant DNA technology relies on a similar

technique that employs an additional step to

isolate plasmids containing the target gene.

To begin, the eukaryotic chromosome and

selected bacterial plasmids to be used as a cloning

vector are treated with a restriction endonuclease.

When the eukaryotic DNA fragments are combined

with the broken plasmids, some of the plasmids

recombine with eukaryotic DNA. The plasmids

are then returned to the host bacteria by simply

culturing both in solution so that some of the

bacteria will take up the plasmids. However, many

of the plasmids will not contain recombinant DNA;

of those that do, only a small portion will contain

the target mammalian gene. Therefore, the next

EXPECTATIONS

Demonstrate an understanding of genetic manipulation, and of its industrial

and agricultural applications.

Outline contributions of genetic engineers, molecular biologists, and

biochemists that have led to the further development of the field of genetics.

Discuss social and ethical issues associated with genetic engineering.