Genetic Engineering Essay, Research Paper
Genetic Engineering
We find it mixed in our food on the shelves in the supermarket,
genetically engineered soybeans and maize. We find it growing in a plot
down the lane, test field release sites with genetically engineered
grape seed, sugar beet, wheat, potato, strawberries and more. There has
been no warning and no consultation.
It is variously known as genetic engineering, genetic modification or
genetic manipulation. All three terms mean the same thing, the
reshuffling of genes usually from one species to another. Existing
examples include; from fish to tomato or from human to pig. Genetic
engineering (GE) comes under the broad heading of biotechnology.
But how does it work? If you want to understand genetic engineering it
is best to start with some basic biology.
What is a cell? A cell is the smallest living unit, the basic
structural and functional unit of all living matter, whether that is a
plant, an animal or a fungus. Some organisms such as amoebae, bacteria,
some algae and fungi are single-celled – the entire organism is
contained in just one cell. Humans are quite different and are made up
of approximately 3,000,000,000,000 cells. Cells can take many shapes
depending on their function, but most commonly they will look like a
brick with rounded corners or an angular blob such as a building block.
Cells are stacked together to make up tissues, organs or structures.
In an organism, cells depend on each other to perform various functions
and tasks, some cells will produce enzymes, and others will store
sugars or fat. Different cells again will build the skeleton or be in
charge of communication like nerve cells others are there for defence,
such as white blood cells or stinging cells in jelly fish and plants.
In order to be a fully functional part of the whole, most cells have
got the same information and resources and the same basic equipment.
A cell belonging to higher organisms, such as a plant or animal, is
composed of a cell membrane enclosing the whole cell. Plant cells
have an additional cell wall for structural reinforcement. The
nucleus is the command centre of the cell. It contains all the vital
information needed by the cell or the whole organism to function, grow
and reproduce. This information is stored in the form of a genetic code
on the chromosomes, which are situated inside the nucleus.
Proteins are the basic building materials of a cell, made by the cell
itself. Looking at them in close-up they consist of a chain of amino
acids and small specific building blocks that easily link up. Though
the basic structure of proteins is linear, they are usually folded and
folded again into complex structures. Different proteins have different
functions. They can be transport molecules such as the oxygen binding
hemoglobin of the red blood cells, or they can be antibodies,
messengers, enzymes or hormones. Another group is the structural
proteins that form boundaries and provide movement, elasticity and the
ability to contract. Muscle fibres, for example, are mainly made of
proteins. Proteins are thus crucial in the formation of cells and in
giving cells the capacity to function properly.
Chromosomes means coloured bodies, as they can be seen under the light
microscope, using a particular colour of stain. They look like bundled
up knots and loops of a long thin thread. Chromosomes are the storage
place for all genetic, that is hereditary, information. This
information is written along the thin thread, called DNA. DNA is an
abbreviation for deoxyribonucleic acid, a specific acidic material that
can be found in the nucleus. The genetic information is written in the
form of a code, almost like a music tape. To ensure the thread and the
information are stable and safe, a twisted double stranded thread is
used, this is the famous double helix . When a cell multiplies it will
also copy all of the DNA and pass it on to the daughter cell.
The totality of the genetic information of an organism is called
genome. Human cells, for example, possess two sets of 23 different
chromosomes, one set from the mother and the other from the father. The
DNA of each human cell corresponds to 2 meters of DNA if it is
stretched out and it is thus crucial to organise the DNA in
chromosomes, so as to avoid knots, tangles and breakages. The length of
DNA contained in the human body is approximately 60,000,000,000
kilometres. This is equivalent to the distance to the moon and back
8000 times!
The information contained on the chromosomes in the DNA is written and
coded in such a way that it can be understood by almost all living
species on earth. It is thus termed the universal code of life. In this
coding system, cells need only four symbols, called nucleotides, to
spell out all the instructions of how to make any protein. Nucleotides
are the units DNA is composed of and their individual names are
commonly abbreviated to the letters A, C G and T. These letters are
arranged in 3-letter words which in turn code for a particular amino
acid. The information for how any cell is structured or how it
functions is all encoded in single and distinct genes. A gene is a
certain segment of DNA with specific instructions for the production of
commonly one specific protein. The coding sequence of a gene is, on
average, about 1000 letters long. Genes code for example for insulin,
digestive enzymes, blood clotting proteins, or pigments.
How does a cell know when to produce which protein and how much of it?
In front of each gene there is a stretch of DNA that contains the
regulatory elements for that specific gene, most of which is known as
the promoter. It functions like a control tower, constantly holding a
flag up for the gene it controls. Take insulin production, which we
produce to enable the burning of the blood sugar. When a message
arrives in the form of a molecule that says, more insulin , the
insulin control tower will signal the location of the insulin gene and
say over here . The message molecule will dock in and thus activate
a switch to start the whole process of gene expression.
How does the information contained in the DNA get turned into a protein
at the right time? Each gene consists of 3 main components: a “control
tower”, an information block and a polyA signal element. If there is
not enough of a specific protein present in the cell, a message will be
sent into the nucleus to find the relevant gene. If the control tower
recognises the message as valid, it will open the “gate” to the
information block. Immediately the information is copied, or
transcribed, into a threadlike molecule called RNA. RNA is very similar
to DNA, except it is single stranded. After the copy is made, a string
of up to 200 “A” type nucleotides, a polyA tail, is added to its end .
This process is called poly-adenylation and is initiated by a polyA
signal located towards the end of the gene. A polyA tail is thought to
stabilize the RNA message against degradation for a limited time. Now
the RNA copies of the gene leave the nucleus and get distributed within
the cell to little work units that translate the information into
proteins.
No cell will ever make use of all the information coded in its DNA.
Cells divide the work up amongst one other – they specialise. Brain
cells will not produce insulin, liver cells will not produce saliva,
nor will skin cells start producing bone. If they did, our bodies could
be chaos!
The same is true for plants. Root cells will not produce the green
chlorophyll, nor will the leaves produce pollen or nectar. Furthermore,
expression is age dependent as young shoots will not express any genes
to do with fruit ripening, while old people will not usually start
developing another set of teeth.
All in all, gene regulation is very specific to the environment in
which the cell finds itself and is also linked to the developmental
stages of an organism. So if I want the leaves of poppy plants to
produce the red colour of the flower petals I will not be able to do so
by traditional breeding methods, despite the fact that leaf alleles
will have all the genetic information necessary. There is a block that
prevents he leaves from going red. This block may be caused by two
things. First of all, the “red” gene could have been permanently shut
down and bundled up thoroughly in all leaf cells. Thus the information
cannot be accessed any more. The second thing is that the leaf cells
may not need the colour red and thus do not request RNA copies of this
information. Therefore no message molecule is docking at the “red”
control tower to activate the gene.
Of course, as you might have guessed, there is a trick to fool the
plant and make it turn red against its own will. We can bring the red
gene in like a Trojan horse, hidden behind the control tower of a
different gene. But for this we need to cut the genes up and glue them
together in a different form. This is where breeding ends and genetic
engineering begins.
Breeding is the natural process of sexual reproduction within the same
species. The hereditary information of both parents is combined and
passed on to the offspring. In this process the same sections of DNA
can be exchanged between the same chromosomes, but genes will always
remain at their very own and precise position and order on the
chromosomes. A gene will, therefore always be surrounded by the same
DNA unless mutations or accidents occur. Species that are closely
related might be able to interbreed, like a donkey and a horse, but
their offspring will usually be infertile, such as the mule. This is a
natural safety devise, preventing the mixing of genes that might not be
compatible and to secure the survival of the species.
Genetic engineering is used to take genes and segments of DNA from one
species, such as fish, and put them into another species, such as
tomatoes. To do so, genetic engineering provides a set of techniques to
cut DNA either randomly or at a number of specific sites. Once isolated
one can study the different segments of DNA, multiply them up and stick
them next to any other DNA of another cell or organism. Genetic
engineering makes it possible to break through the species barrier and
to shuffle information between completely unrelated species. A good
example of this would be to splice the anti-freeze gene from flounder
into tomatoes or strawberries, or an insect-killing toxin gene from
bacteria into maize, genes from humans into pig.
Yet there is a problem. A fish gene will not work in tomato unless I
give it a promoter with a “flag” the tomato cells will recognise. Such
a control sequence should either be a tomato sequence or something
similar. Most companies and scientists do a shortcut here and don’t
even bother to look for an appropriate tomato promoter as it would take
years to understand how the cell’s internal communication and
regulation works. In order to avoid long testing and adjusting, most
genetic engineering of plants is done with viral promoters. Viruses are
very active. Nothing, or almost nothing, will stop them once they have
found a new victim, or host. They integrate their genetic information
into the DNA of a host cell, multiply, infect the next cells and
multiply. This is possible because viruses have evolved very powerful
promoters that command the host cell to constantly read the viral genes
and produce viral proteins. Simply by taking a control element from a
plant virus and sticking it in front of the information block of the
fish gene, you can get this combined virus/fish gene, known as a
“construct’, to work wherever and whenever you want in a plant.
This might sound great, the drawback though is that it can’t be stopped
either, it can’t be switched off. The plant no longer has a say in the
expression of the new gene, even when the constant involuntary
production of the “new” product is weakening the plant’s defences or
growth.
And furthermore, the theory doesn’t hold up with reality. Often, for no
apparent reason, the new gene only works for a limited amount of time
and then “falls silent”. Yet there is no way to know in advance if this
will happen.
Though often hailed as a precise method, the final stage of placing the
new gene into a receiving higher organism is rather crude, seriously
lacking both precision and predictability. The “new” gene can end up
anywhere, next to any
gene or even within another gene, disturbing its function or
regulation. If the “new” gene gets into the “quiet” non-expressed areas
of the cell’s DNA, it is likely to interfere with the regulation of
gene expression of the whole region. It could potentially cause genes
in the “quiet” DNA to become active.
Often genetic engineering will not only use the information of one gene
and put it behind the promoter of another gene, but will also take bits
and pieces from other genes and other species. Although this is aimed
to benefit the expression and function of the “new” gene it also causes
more interference and enhances the risks of unpredictable effects.
There are different ways to get a gene from A to B or to transform a
plant with a “new” gene. A vector is something that can carry the gene
into the host, or rather into the nucleus of a host cell. Vectors are
commonly bacterial plasmids or viruses. Another method is the “shotgun
technique”, also known as “bio-ballistics,” which blindly shoots masses
of tiny gold particles coated with the gene into a plate of plant
cells, hoping to land a hit somewhere in the cell’s DNA.
Plasmids can be found in many bacteria and are small rings of DNA with
a limited number of genes. Plasmids are not essential for the survival
of bacteria but can make life a lot easier for them. Whilst all
bacteria, no matter which species, will have their bacterial chromosome
with all the crucial hereditary information of how to survive and
multiply, they invented a tool to exchange information rapidly. If one
likens the chromosome to a bookshelf with manuals and handbooks, and a
single gene to a recipe or a specific building instruction, then a
plasmid could be seen as a pamphlet. Plasmids self-replicate and are
thus easily reproduced and passed around. Plasmids often contain genes
for antibiotic resistance. This type of information can be crucial to
bacterial strains which are under attack by drugs and is indeed a major
reason for the quick spread of antibiotic resistance.
Plasmids are relatively small, replicate very quickly and are thus easy
to study and easy to manipulate. It is easy to determine the sequence
of its DNA, that is, to find out the sequence of the letters and number
them. Certain letter combinations such as CAATTG are easy to cut with
the help of specific enzymes. these cutting enzymes, called restriction
enzymes, are part of the genetic engineering “tool-kit” of biochemists.
So if I want to splice a gene from fish into a plasmid, I have to take
the following steps: I place a large number of a known plasmid in a
little test tube and add a specific enzyme that will cut the plasmid at
only one site. After an hour I stop the digest, purify the cut plasmid
DNA and mix it with copies of the fish gene. After some time the fish
gene places itself into the cut ring of the plasmid. I quickly add some
“glue” from my “tool-kit”, an enzyme called ligase, and place the
mended plasmids back into bacteria, leaving them to grow and multiply.
How do I know which bacteria will have my precious plasmid? For this
reason I need marker genes, such as antibiotic resistance genes. So I
make sure my plasmid has a marker gene before I splice my fish gene
into it. If the plasmid is marked with a gene antibiotic resistance I
can now add specific antibiotic to the food supply of the bacteria. All
those which do not have the plasmid will die, and all those that do
have the plasmid will multiply.
Genetic Engineering is a test tube science and is prematurely applied
in food production. A gene studied in a test tube can only tell what
this gene does and how it behaves in that particular test tube. It
cannot tell us what its role and behaviour are in the organism it came
from or what it might do if we place it into a completely different
species. Genes for the colour red placed into petunia flowers not only
changed the colour of the petals but also decreased fertility and
altered the growth of the roots and leaves. Salmon genetically
engineered with a growth hormone gene not only grew too big too fast
but also turned green. These are unpredictable side effects,
scientifically termed pleiotropic effects.
We also know very little about what a gene, or for that matter any of
its DNA sequence, might trigger or interrupt depending on where it got
inserted into the new host. These are open questions around positional
effects. And what about gene silencing and gene instability? How do we
know that a genetically engineered food plant will not produce new
toxins and allergenic substances or increase the level of dormant
toxins and allergens? How about the nutritional value? And what are the
effects on the environment and on wild life? All these questions are
important questions yet they remain unanswered. Until we have an answer
to all of these, genetic engineering should be kept to the test tubes.