Page 36 - 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

240 Biopolymers

of protein synthesis taking place from the m-RNA. Some the viral outer covering, and stimulation of antibodies in
proteins involved in enhancing transcription or initiating human beings against coat proteins can provide protec-
translation can themselves be activated as the result of a tion against viral diseases. Where a high mutation rate in
reaction cascade stimulated by a messenger molecule such viral DNA causes fast changes in coat protein amino acid
as a hormone (see Section II.A.2), thus allowing another sequences, it becomes difficult to produce a successful
form of control. vaccine, since antibodies against the original coat protein
Correct replication of DNA, transcription to RNA, and maybeineffectiveagainstanewalteredprotein.Inthecase
translation to amino acid sequences must be carried out if of the virus, HIV, that causes AIDS, the mutation rate is
fully functional proteins are to be synthesized. A change unusually high, making for great difficulty in producing a
of one base on a DNA or m-RNA can change a codon, useful vaccine.
so that an incorrect amino acid is inserted into a protein. Antibiotics are compounds synthesized by some mi-
Such a change in DNA is known as a mutation. For exam- croorganisms, with the property of inhibiting the growth
ple, a change of GAA to GUA would substitute valine for of others. Many of these act by interfering with nucleic
glutamic acid in a protein. This kind of change is known acid function. Thus actinomycin D and mytomycin C bind
in human hemoglobin where substitution of a valine for to DNA and inhibit replication; streptomycin and the tetra-
a particular glutamic acid causes a change in the surface cyclines bind to ribosomes and inhibit translation, while
properties of the hemoglobin molecules and produces a puromycin, by its structure, mimics that end of a t-RNA
disease known as sickle cell anemia. At the level of DNA which accepts an amino acid and brings about release of
replication, DNA polymerases can “proofread” the new an incomplete polypeptide from a ribosome. AZT (azi-
DNA, remove mismatched bases, and incorporate correct dothymidine), a modified form of a thymine nucleoside,
bases into the growing polynucleotide strand. Other en- inhibits the reverse transcriptase of HIV and hence is use-
zymes can repair some damage to existing DNA. Thus ful in slowing down the development of AIDS.
mutations caused by errors in replication are rare, approx-
11
imately 1 in 10 bases. Lack of certain repair enzymes in
3. Utilization—Recombinant DNA
human beings is known to be associated with particular
and Genetic Engineering
kinds of cancer. RNA polymerases, on the other hand, do
not have proofreading ability and so the rate of incorpora- Many medically important proteins, for example, human
tion of mismatched bases is higher during RNA synthesis. insulin and human somatotropin (a growth hormone) are
Other mutations can cause base additions or deletions in short supply. Now that the mechanisms of protein syn-
in DNA, which can drastically alter protein structure, r- thesis are understood, it is possible to introduce a segment
RNA or t-RNA structure or control systems, depending on of DNA, coding for say a human protein, into bacteria,
the location of the alteration on the DNA. Mutations can culture the bacteria, allow them to synthesize the protein,
occur spontaneously or by the action of chemicals. Some and then harvest that protein. Such a process involves re-
of these mutagenic chemicals are also carcinogens (i.e., combinant DNA technology.
cancer-causing agents). First, the DNA segment which codes for the required
In animals, certain viruses have been implicated in the protein must be prepared. This may be purified from cells
development of cancer. These tumor-producing viruses in which it occurs, by partial degradation of the total cel-
can have DNA or RNA as their genetic material. Most lular DNA, and isolation of the appropriate fragment. Al-
RNA viruses contain an enzyme, reverse transcriptase, ternatively, if purified m-RNA for the protein is available,
which is capable of synthesizing DNA on an RNA tem- the corresponding DNA can be made using reverse tran-
plate. Both types of virus can bring about incorporation of scriptase. If the DNA segment required is short, a small
viral DNA into the animal cell DNA, thus altering the ge- amount can be synthesized chemically in a laboratory, and
netic makeup of the cell. Some of the viral DNA may con- then greater quantities can be made by enzymic replication
stitute an oncogene—a gene coding for a protein related, in the so-called polymerase chain reaction (PCR).
but not identical to, a normal cellular protein. The normal Second, this DNA must be incorporated into a “vec-
protein is often involved in control of cell growth, and the tor” which will carry the DNA into the bacterial cells.
production of the altered protein from the oncogene may The vector may be the DNA of a virus which can invade
bring about transformation of the animal cells to malig- the strain of bacterium to be used. More commonly, the
nant cells. Reverse transcriptases, like RNA polymerases, foreign DNA is introduced into a plasmid, a small circu-
do not proofread the new nucleic acid strand being synthe- lar piece of DNA which can be replicated in the bacterial
sized, and so a high mutation rate in viral DNA is possible. cells. The circular plasmid DNA is usually “nicked” by
This can be important for human health. Some viral DNA an enzyme, and another enzyme is used to join in the
codes for proteins, the so-called coat proteins, that form foreign DNA. If the cut ends of the plasmid DNA and the