Page 389 - Carrahers_Polymer_Chemistry,_Eighth_Edition

352 Carraher’s Polymer Chemistry

material entails the use of polymer chemistry on a molecular (or nano) level making use of some-
what straightforward chemical reactions, many of the reactions employing biological entities, such
as enzymes, to carry out these reactions.
Essentially, gene segments are replaced to inject into the altered microorganism genetic material
that expresses the desired trait. Today, routine gene alteration is taught in undergraduate laborato-

ries. Even so, specific gene alteration requires extensive planning and is conducted in major research
laboratories.

In the broadest sense, genetic engineering refers to any artificial process that alters the genetic
composition of an organism. Such alterations can be carried out indirectly through chemical meth-
ods, through radiation, or through selective breeding. Today, the term usually refers to the process
whereby genes or portions of chromosomes are chemically altered.
After the alteration of a single, or few, genes, the altered genes reproduce giving much larger
numbers of genes with the alternation incorporated in the their genome. The term “clone” comes
from the Greek work klon, meaning a cutting used to propagate a plant. Cell cloning is the produc-
tion of identical cells from a single cell. In like manner, gene cloning is the production of identical
genes from a single gene, introduced into a host cell. Today, the term cloning refers to one special
type of genetic engineering.
Genes are a chromosomal portion that codes for a single polypeptide or RNA. Gene splicing
is currently practiced as the enzymatic attachment of one gene or gene segment to another gene
or gene segment. Genes are composed of DNA, which can be considered as a specialized poly-
phosphate polymer. The manipulation of DNA can occur for many reasons. One of these is the
production of recombinant DNA. Here we will focus on the production of recombinant DNA. DNA
cannot be directly transferred from one organism, the donor, to another recipient organism, the host.
Instead, the donor DNA segment is cut and then recombined with a DNA from a host. E. coli is typ-
ically employed as the host cell since it is itself a harmless bacterium that reproduces rapidly. (But
under the wrong conditions E. coli is responsible for many food poisonings.) The E. coli then acts
as a “factory” that reproduces bacteria that contain the desired modifi cation.
Enzymes, specialized proteins, are used as designing tools for the genetic engineering. One of
these enzyme tools consists of restriction endonucleases that recognize a specific series of base

pairs. They split the DNA at these specific points. This splitting is called “lysing,” which in reality

is simply the hydrolysis of DNA units as shown below:
R O R O
Restriction endonuclease
−
O P O O P OH + HO R (10.10)
O − R O − R

Organisms produce restriction endonucleases that are specific for that organism. Certain restric-

tion endonucleases cut double-stranded DNA asymmetrically in regions called palindromes, that is
regions that “read” (have identical sequences) the same way from left to right on one strand as right
to left on the other strand. This produces what is referred to as “sticky ends” that form not only a
“cleft” for attachment but also a single-stranded end that has the ability to pair with another compli-
mentary single-stranded strand end. Both strands of the original donor twin strand have a tendency
to recombine with complementary strands of DNA from a host that has been treated to produce the
complementary strands. The sticky ends, when mixed under the proper conditions in the presence
of another enzyme, DNA-ligase, combine. The hydrogen bonding between complementary sticky
ends reenforce the recombination reaction. The resulting recombination reaction results in a variety
of products, including the desired recombination of host and donor DNA as well as the combination
of the original donor strands and uncombined DNA. The mixture is often treated in one of two man-
ners. The simplest case requires a chemical-resistant gene that is resistant to the employed chemical

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