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Naturally Occurring Polymers—Animals 353

agent, such as tetracycline. The desired recombinant genes survive and are then transferred into the
host organism so the new gene can express itself.
In some cases, such as the synthesis of insulin, the recombination mixture is added to a host
organism, here E. coli. This infected mixture is then plated out and the individual colonies tested
for insulin production. Those colonies that produce insulin are further plated out and grown for
mass insulin production. Cells that accept the recombinant DNA are called transformed. More spe-
cialized sequences have been developed to increase the probability of gene incorporation and its
successful reproduction.
A second tool employed by the genetic engineer is the enzyme terminal transferase that adds
deoxyribonuclease resides to the 3′ end of DNA strands creating 3′ tails of a single type of residue.
Special modifi ed plasmid DNA’s, called vectors or carriers, are used as host or targets for gene

modification. These circularly shaped vectors reproduce autonomously in host cells. Plasmids have two
other important properties. First, they can pass from one cell to another allowing a single “modifi ed”

bacterial cell to inject neighboring bacterial cells with this “modification.” Second, gene material from
other cells can be easily formed into plasmids, allowing ready construction of modifi ed carriers.
The steps involved in gene splicing, emphasizing the chemical nature of the individual steps, are
as follows:
1. Lysing (which is really simply the hydrolysis of DNA units as shown above)
2. Construction of staggered, sticky, ends
3. Recombination or lysation, the reverse of lysing, chemically formation of a phosphate ester as
below connecting the desired segment to the DNA of the host cell

R O R O
R 1 OH + O P OH O P O + H 2 O (10.11)

O − R O − R 1

4. Chemical recombination of vector-insertion into the host cell; recombining plasmid genes
into the host genetic complement
5. Replication of host cell

There are many uses of recombinant DNA. As noted above, one technique that produces recom-
binant DNA is called cloning. In one cloning technique used for the production of the sheep Dolly
in 1996, the DNA nucleus from a female’s egg is replaced with a nucleus from another sheep. The
egg is placed in the uterus of a third animal, known as the surrogate mother. Dolly is nearly geneti-
cally identical to the animal from which the nucleus was obtained but not genetically related to the
surrogate mother.
Recombinant DNA has been used in a variety of ways. The growth hormone gene of rainbow
trout has been transferred into carp eggs resulting in the transgenic carp producing larger fi sh. The
milk production of dairy cows has been increased by cloning and introducing into the cows the
cattle growth hormone bovine somatotropin.
Transgenic strawberry and potato plants have been produced that are frost-resistant. Cotton,
corn, soybean plants have been produced with increased resistance to herbicides allowing herbicide
use without killing the transgenic crop-producing plants. Larger and smaller varieties of other food-
producing plants have been produced using recombinant DNA as have plants that produce certain
amino acids needed for our nutrition.
Transgenic bacteria have been produced that can metabolize petroleum products, including cer-
tain synthetic polymers.

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