Page 37 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology EN002C-64 May 19, 2001 20:39

Biopolymers 241

foreign DNA have complementary sequences, then correct for a virus coat protein into a plant. This appears to stop
combination of the two is easier. These so-called “sticky” the virus replicating in the plant. The first commercially
ends can be produced by enzymes or chemical synthe- grown genetically modified plants contain DNA coding
sis. The product—an artificial combination of bacterial for an enzyme that confers herbicide resistance or for pro-
plasmid DNA and, for example, human DNA—is called teins that are insecticidal. In the first case, less herbicide
recombinant DNA. need be used on crops to kill weeds, as spraying can be
The plasmid is then introduced into a bacterial cell and carried out when both the weeds and crop plants are well-
the bacterium is cloned (i.e., the cell is allowed to divide grown. In the second case, application of external pesti-
several times) producing progeny bacteria with the same cide may not be required, since the crop plants themselves,
genetic constitution as the original cell. Each cell carries containing the new insecticidal protein, may kill invading
the DNA coding for the required protein, and if the DNA insects.Currently,cotton,soya,maize,andcanolacontain-
has been inserted with the correct control elements, each ing these modifications are grown commercially, but be-
cell synthesizes the protein. The protein can later be puri- cause of concerns about possible environmental problems
fied from the bacterial culture. such as potential harm to beneficial insects, introduction of
In this way it has been possible to produce human in- new genetically-engineered crop plants has slowed down.
sulin, growth hormone, and interferon (an antiviral pro- An extension of this work could lead to “genetic en-
tein) from E. coli—a common bacterium in the human gineering” in human beings, where genetic diseases (i.e.,
digestive tract. diseases caused by a defective or missing enzyme) might
Fears that new and dangerous strains of E. coli might be cured by incorporation of DNA coding for that pro-
result from recombinant DNA experiments have mostly tein. It will be difficult, however, to ensure that subsequent
been allayed by stringent controls on the kinds of experi- biosynthesis of the protein is under correct control, so that
ments that can be carried out and by greater understanding the protein is made in the right amounts in the appropriate
of the control mechanisms involved in bacterial nucleic tissues.
acid metabolism. Small amounts of human DNA found at the scene of
It is hoped by these methods to produce medically use- a crime can now be amplified by PCR technology, and
ful proteins cheaply and on a large scale. In addition, new the DNA can be hydrolyzed by enzymes specific for par-
bacteria with useful properties may be “created” (e.g., bac- ticular base sequences. Because the pattern of fragments
teria able to convert cellulose to methane for energy pro- produced is believed to be characteristic of one human
duction). (At present two different microorganisms must individual, the process is now used to help identify perpe-
be used—one converts cellulose to acids, the second these trators of serious crimes.
acids to methane.)
By incorporating DNA with specific base changes into
D. Rubber, Lignin, and Polyesters
bacteria it is possible to obtain proteins with as little
as one altered amino acid. This process, known as site- Rubber is a hydrocarbon polymer produced by many trop-
directed mutagenesis, has enabled detailed investigations ical and a few temperate plants, such as the dandelion.
to be made of the role of individual amino acids in, for Commercially, rubber is obtained from a tree originally
example, enzyme activity and stability. It is hoped that, in found in Brazil, Hevea brasiliensis, which is now grown
the future, industrially important enzymes with improved extensively in southeast Asia and Africa, as well as Cen-
thermal stability may be made in this way. tral and South America. Rubber is obtained from the tree
Genetic engineering can also be carried out on plants in the form of latex, a suspension of polymer particles in
and animals, although there is usually some difficulty in a slightly viscous aqueous solution; this latex oozes from
introducing the new DNA into the plant or animal cell the tree when a cut is made in the bark. Trees may be
and ensuring that it is used to code for a new protein. tapped again and again for more than 30 years, and com-
Single plant cells can be transformed by infection with a mercial yields can be more than 3300 pounds per acre per
bacterium, Agrobacterium tumefaciens, carrying a plas- year (3600 kg/ha/yr).
mid that contains the desired new DNA. When the bac- The monomer of rubber is isopentenyl pyrophosphate,
terium contacts a plant cell, some of the plasmid DNA is and when the polymer is synthesized, the monomers be-
transferred to the plant cell nucleus and is integrated into come linked together, with the elimination of pyrophos-
the plant cell’s DNA. Then whole plants can be produced phate, as in Fig. 23. During the joining of the monomer
from the single plant cells in culture by the administra- residues, the carbon-carbon double bond changes position
tion of hormones. Thus, it is possible to grow complete (from left of the CH 3 group, to right of it) and the repeat
plants with an altered genetic makeup. Disease resistance unit in the polymer is an isoprene unit; rubber is, thus,
can be improved, for example, by incorporating a gene a polyisoprene. In latex, the polyisoprene molecules are