Genetics Essay, Research Paper
GENETIC ENGINEERING
“career of the future”
Genetic engineering is an umbrella term that can cover a wide range of ways of
changing the genetic material — the DNA code — in a living organism. This code contains
all the information, stored in a long chain chemical molecule, which determines the
nature of the organism. Apart from identical twins, genetic make-up is unique to each
individual. Individual genes are particular sections of this chain, spaced out along it,
which determine the characteristics and functions of our body. Defects of individual
genes can cause a malfunction in the metabolism of the body, and are the roots of many
?genetic? diseases.
In a sense, man has been using genetic engineering for thousands of years. We
weren’t changing DNA molecules directly, but we were guiding the selection of genes.
For example the domestication of plants and animals.
Recombinant DNA technology is the newest form of genetic engineering, which
involves the manipulation of DNA on the molecular level. This is a totally new process
based on the science of molecular biology, a relatively new science only forty years old.
It represents a major increase in our ability to improve life. But a negative aspect is that it
changes the forms of life we know of, possibly damaging our environment
It has been known for some time that genetic information can be transferred
between micro-organisms. This is process it done via plasmids (small circular rings of
DNA) or phages (bacterial viruses). Both of these are termed vectors, this is because of
their ability to move genetic material. In general this is limited to simpler species of
bacteria. nevertheless, this can restriction can be overcome with the use of genetic
engineering because it allows the introduction of any gene.
While genetic engineering is beginning to be used to produce enzymes, the
technology itself also depends on the harnessing of enzymes, which are available in
nature. In the early 1970s Herbert Boyer, working at the University of California Health
Science Centre in San Francisco, and Stanley Cohen at Stanford University found that it
was possible to insert into bacteria genes they had removed from other bacteria. First
they learned the trick of breaking down the DNA of a donor organism into manageable
fragments. Second, they discovered how to place such genes into a vector, which they
used to ferry the fragments of DNA into recipient bacteria. Once inside its new host, a
transported gene divided as the cell divided, leading to a clone of cells, each containing
exact copies of the gene. This technique became known as gene cloning, and was
followed by the selection of recipient cells containing the desired gene.
The enzymes used for cleaving out the DNA pieces act in a highly specific way.
Genes can, therefore, be removed and transferred from one organism to another with
extraordinary precision. Such manoeuvres contrast sharply with the much less predictable
gene transfers that occur in nature. By mobilising pieces of DNA in this way (including
copies of human genes), genetic engineers are now fabricating genetically modified
microbes for a wide range of applications in industry, medicine and agriculture.
The underlying idea of transferring genes between cells is quickly explained.
However the actual practice is an extremely complicated process. The scale of the
problem can be gauged from the astronomical numbers involved: the DNA of even the
simplest bacterium contains 4,800,00 pairs of bases. But there is only one copy of each
gene in each cell.
First, restriction enzymes are used to snip the DNA into smaller pieces, each
containing one or just a few genes. These enzymes cut DNA in very precise ways. They
recognise particular stretches of bases (termed recognition sequences) and snip each
strand of the double helix at a particular place. Whenever the recognition sequence
appears in the long DNA chain, the enzyme makes a cut. Whenever the same enzymes
are used to break up a certain piece of DNA, they always produce the same set of
fragments. The cuts produce pieces of double helix with short stretches of single
stranded DNA at each end. These are know as sticky ends. If the enzyme is allowed to act
for a limited time, it may not have a chance to attack all the recognition sequences in the
chain. This will result in longer fragments.
As in natural DNA replication, bases have an inherent propensity to join up with
their partners A with T, for example, and G with C. So too with sticky ends. For
example, the sequence TTAA will tend to re-associate with AATT. Genetic
engineers use another type of enzyme, DNA ligase, to make the union permanent. This is
the key principle of genetic engineering the use of two types of enzyme to cut out one
piece of DNA and then to attach it to another piece. The genetic engineer’s toolkit now
contains several hundred different restriction enzymes. Each is a precision instrument for
fragmenting DNA in a particular way. Some recognise different base sequences; others
recognise the same sequence but snip at a different point within or next to the sequence.
Ferrying DNA to a new home Once a piece of DNA has been broken up into a mixture of
fragments, these can be separated into different sized pieces. The next stage is to insert a
particular DNA fragment into a vector. Often this is a plasmid, a selfreplicating circular
piece of DNA that can become incorporated in the bacterial nucleus and later become
detached, carrying genes with it. Plasmids seem to have evolved as a natural mechanism
for moving genes around among bacteria.
To insert a DNA fragment into a vector, the genetic engineer first splits open the
plasmid by adding the same restriction enzyme that was used to release the DNA
fragment from the DNA of the donor organism. This creates sticky ends complementary
to those on the fragment to be transplanted. The fragment thus fits neatly into the gap in
the vector DNA, where it is firmly annealed by DNA ligase.
Next, the plasmid is allowed to infect a bacterium, in which it can replicate. Once
inside, the vector and thus the foreign gene replicates every time the cell divides. As
bacteria divide about once every 20 minutes, gene cloning can lead to a billionfold
increase in the number of copies of a particular gene within 10 hours or so. The
bacterium simply treats it and replicates it as part of its own DNA.
Not all the countless cells in a culture of bacteria become infected when a vector
is added. One method of distinguishing those that do contain the vector is to incorporate
into it a gene that confers resistance to a particular antibiotic. When the bacteria are
cultured later, that antibiotic is included in the nutrient medium to inhibit any non
resistant organisms. Only bacteria that have taken up the vector (and thus the resistance
gene) are able to grow. A similar trick distinguishes bacteria carrying the vector plus a
new gene from unwanted ones containing the unaltered vector.
By using a variety of restriction enzymes to cut up DNA into manageable pieces,
and then cloning these sequences, it is possible to create a DNA library a collection of
sequences carrying all the genetic information in a particular organism. But much of this
information is not expressed at any particular moment. Genetic engineers are usually
interested only in the genes that are actually functioning at any one time for example,
one responsible for producing a specific enzyme. The DNA that codes for hereditary
messages specifying current activities of this sort is much smaller in quantity than the
total DNA in a cell. This information is to be found in messenger RNA. An enzyme
called reverse transcriptase allows its messages to be translated into DNA. This copy
DNA (cDNA) is then cloned into bacteria, giving a library, much smaller than that of a
cell’s total DNA, that will certainly contain the desired gene. But this still leaves the final
challenge of locating the specific bacteria containing the spliced gene. One method is to
spread the bacteria infected with the vector onto a nutrient medium, on which each
individual cell can spawn millions of progeny and thus appear as a visible colony. The
genetic engineer also needs to know the amino acid sequence of the protein coded by the
gene. By following the genetic code, a corresponding stretch of RNA can now be
synthesised chemically. During the synthesis, radioactive atoms are incorporated into the
RNA, making a gene probe.
The next step is to make, on special filter paper, a replica of the plate with the
colonies of the cloned bacteria. Treated with caustic soda, the bacteria burst open and
release their DNA, which is also broken into single strands that stick to the filter. The
gene probe is now added. If the correct sequence is present, the probe will pair
tenaciously with it. The filter is now washed to remove the unbound probe, and placed
over a piece of x-ray film. When developed, the film reveals the location of the
radioactivity as a black spot. The corresponding colony on the original plate thus
contains the bacteria carrying the required gene.
The applications of genetic engineering are vast, probably the most well known is
gene therapy in the medical world. It involves the introduction of a gene into somatic
cells and enablement of its products to alleviate a disorder caused by the loss or
malfunctioning of a vital gene product. Involving the latest DNA technology, it is the
most rapidly advancing form of molecular medicine, which is concerned with the cause
of disease at a molecular level. The scope for gene therapy has increased over in the last
few years with the possibility of a therapeutic gene for diseases such as cancer, AIDS,
cystic fibrosis, and even neurological disorders such as Parkinson’s disease and
Alzheimer’s disease.
The potential of gene therapy to treat specific human diseases, has hardly become
apparent yet but it is believed be the way forward in the treatment of many diseases.
Trials in United States are being carried out in an attempt to treat AIDS. The strategies
are in the form of a treatment which will protect susceptible cells from infection by the
virus once it is in the body, or to inhibit the replication of HIV in already affected cells.
Moreover to try to boost the immune response to HIV and HIV-infected cells. This and
many other diseases have become to show potential of being treated in this fashion.
Gene therapy has resulted in the possible reduction in cancerous tumours.
Tumours in lung cancer patients shrunk or stopped growing when scientists inserted
healthy genes into to replace defective or missing genes, it demonstrated that by
correcting a single genetic abnormality in lung cancer cells may be enough to slow down
or stop the spread of cancer. Further research into the use of gene therapy to cure or help
cancer victims has been continued after the discovery of this method.
As well as in medicine there are many applications of genetic engineering in
agriculture. Genetically engineered hormones are available, and may be used in the
future to increase meat or milk yields of livestock. Soon disease may be wiped out with
the use of genetically engineered vaccines. Fertilisers may become obsolete, as scientists
attempt to introduce ntirogenase genes into plants, the gene coding for the enzyme that
catalyses the breakdown of atmospheric nitrogen. Plants could also in theory be able to
produce their own insecticides thus making artificial ones obsolete. Crops could even be
engineered to grow in naturally inhospitable areas and could effectively make food
shortages a thing of the past.
Recently, genetic technology has shown that it will affect our everyday lives, such
as in the grocery store. There has been work in the growing of genetically engineered
foods. The government has even approved the sale of certain products. The nutritional
value can be increased, as well as the hardiness of crops.
Another interesting idea is that of transgenic animals. Transgenic technology
bypasses conventional breeding by using artificially constructed parasitic genetic
elements as vectors to multiply copies of genes, and in many cases, to carry and smuggle
genes into cells that would normally exclude them. (Parasites, by definition, require the
host cell’s biosynthetic machinery for replication.). Once inside cells, these vectors slot
themselves into the host genome. In this way, transgenic organisms are made carrying the
desired transgenes. The insertion of foreign genes into the host genome has long been
known to have many harmful and fatal effects including cancer; and this is borne out by
the low success rate of creating desired transgenic organisms. Typically, a large number
of cells, eggs or embryos have to be injected or infected with the vector to obtain a few
organisms that successfully express the transgene(s).
The most common vectors used in gene biotechnology are a mosaic
recombination of natural genetic parasites from different sources, including viruses
causing cancers and other diseases in animals and plants, with their pathogenic functions
‘crippled’, and tagged with one or more antibiotic resistance ‘marker’ genes, so that cells
transformed with the vector can be selected. For example, the vector most widely used in
plant genetic engineering is derived from a tumour-inducing plasmid carried by the
bacterium Agrobacterium tumefaciens. In animals, vectors are constructed from
retroviruses causing cancers and other diseases. Unlike natural parasitic genetic elements
that have varying degrees of host specificity, vectors used in genetic engineering are
designed to overcome species barriers, and can therefore infect a wide range of species.
Thus, a vector currently used in fish has a framework from the Moloney murine
leukaemic virus, which causes leukaemia in mice, but can infect all mammalian cells. It
has bits from the Rous Sarcoma virus, causing sarcomas in chickens, and from the
vesicular stomatitis virus, causing oral lesions in cattle, horses, pigs and humans.
Genetic fingerprinting is a well-known application of genetic engineering, it is
often used in an aid to identify the perpetrator of a crime. This is possible because
everyone (except identical twins) has a unique genetic fingerprint. The process was
developed by Alecs Jeffreys at the University of Leicester in 1984. He noticed that there
were unusual sequences in DNA that seemed out of place. These sequences
(minisatellites) are repeated many times throughout DNA. A DNA probe is used to
analyse these patterns. A DNA probe is a synthetic length of DNA made up of a repeated
sequence of bases. This is cloned to make a batch of probes using the recombinant DNA
into E. Coli bacterium technique. A radioactive label is then attached by exchanging all
the phosphate molecules with radioactive isotopes of the same species. The DNA which
is to analysed is then fragmented using a restriction enzyme, placed on agarose gel and
the fragments separated using a process called electrophoresis. Fragments of DNA have
negative charges, so when and anode is placed at the other end of the gel, the DNA is
attracted to it. The distances they move are dependent on the size of the fragment, with
the lighter, shorter fragments moving the furthest. Once they are separated, the fragments
are transferred to a nylon membrane are treated with the DNA probe. These bind to any
complementary minisatellite sites, and make them show up on x-ray film because of the
radioactivity. The pattern of bands revealed is known as the DNA fingerprint. This would
seem fail safe, but there are many problems associated with this technique. Samples
taken from the victim’s body will more than likely have the victims DNA as well, not to
mention any bacterial or fungal DNA present. Dyes used in clothes can also alter
restriction enzymes, making them fragment in the wrong place. DNA fingerprinting is
therefore not infallible.
People rightly fear that what they eat could harm them if it has been gene altered.
It is also quite possible that products can be made safer and less allergenic than before
this new technology. If food can be grown more economically as a product of gene
technology, world hunger can be virtually stamped out.
It is feared by some people that we might knock nature off balance by interfering
with it. There is no possible way that it could truthfully be said that we haven’t done so
already. Ever since we discovered how to make fire, we have defeated nature’s balance. It