What is Genetic Engineering?
Written by: Dr. Ricarda Steinbrecher
WEN Trust, July 1998
Synthesis/Regeneration: A Magazine of Green Social Thought, Vol. 18 (Winter
1999), pp. 9-12
[Note: For technical reasons, the graphics accompanying the orginal article
have not been reproduced here.]
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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 rape 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 million
cells -(3,000,000,000,000 cells). Cells can take many shapes depending on
their function, but commonly they will look like a brick with rounded comers
or an angular blob - a building block.Cells are stacked together to make up
tissues, organs or structures (brain, liver, bones, skin, leaves, fruit
etc.).
In an organism, cells depend on each other to perform various functions and
tasks; some cells will produce enzymes, 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 (e.g. plant or animal) is composed of:
· a cell MEMBRANE enclosing the whole cell. (Plant cells have an
additional cell wall for structural reinforcement.)
· many ORGANELLES, which are functional components equivalent to the
organs in the body of an animal e.g. for digestion, storage, excretion.
· a NUCLEUS, 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,
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 (e.g. oxygen binding haemoglobin of the red blood
cells); they can be antibodies, messengers, enzymes (e.g. digestion enzymes)
or hormones (e.g. growth hormones or insulin). 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" (they can be seen under the light
microscope, using a particular 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 deoxyribo
nucleic 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 - the famous double helix. When a
cell multiplies it will also copy all the DNA and pass it on to the daughter
cell.
The totality of the genetic information of an organism is called genome.
Cells of humans, 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 chromo-somes 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 - as shown in the flow diagram 1. 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 (length)
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? As shown in picture 2, each gene consists of 3 main
components: a "control tower" (promoter), 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 (picture 2).
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 stabilise
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: young shoots will not express any genes to do
with fruit ripening, while old people will not usually start developing
another set of teeth (exceptions have been known).
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 f 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 ells 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:
· The "red" gene has been permanently shut down and bundled up
thoroughly in all leaf cells. Thus the information cannot be accessed any
more.
· The leaf cells do 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 - 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 thus 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 (e.g.
mule). This is a natural safety device, preventing the mixing of genes that
might not be compatible and to secure the survival of the species.
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