Genetic Engineering

Topic Notes

Additional Support Materials

i.e. animations, quizzes, pictures,  worksheets

Genetic Engineering


   Electrophoresis - Create a DNA Fingerprint  (provided by: pbs) 

PCR -  Interactive
(provided by: dolan DNA learning center)

PCR Fact Sheet 
(Nat. Human Genome Research Institute)

Southern Blotting (DNA fingerprinting) - interactive animation 
(provided by: dolan DNA learning center)

Gene Technology - sample pages (Philip Allan workbook)

Genes & G.E. Quiz
(provided by:

Applications of Genetic Engineering




Genetic Engineering 
[back to top]

Genetic engineering, also known as recombinant DNA technology, means altering the genes in a living organism to produce a Genetically Modified Organism (GMO) with a new genotype. Various kinds of genetic modification are possible: inserting a foreign gene from one species into another, forming a transgenic organism; altering an existing gene so that its product is changed; or changing gene expression so that it is translated more often or not at all.


Techniques of Genetic Engineering [back to top]

Genetic engineering is a very young discipline, and is only possible due to the development of techniques from the 1960s onwards. Watson and Crick have made these techniques possible from our greater understanding of DNA and how it functions following the discovery of its structure in 1953. Although the final goal of genetic engineering is usually the expression of a gene in a host, in fact most of the techniques and time in genetic engineering are spent isolating a gene and then cloning it. This table lists the techniques that we shall look at in detail.  





To make a DNA copy of mRNA


Restriction Enzymes

To cut DNA at specific points, making small fragments


DNA Ligase

To join DNA fragments together



To carry DNA into cells and ensure replication



Common kind of vector


Gene Transfer

To deliver a gene to a living cells


Genetic Markers

To identify cells that have been transformed


Replica Plating *

To make exact copies of bacterial colonies on an agar plate



To amplify very small samples of DNA


DNA probes

To identify and label a piece of DNA containing a certain sequence


Shotgun *

To find a particular gene in a whole genome


Antisense genes *

To stop the expression of a gene in a cell


Gene Synthesis

To make a gene from scratch



To separate fragments of DNA

 * Additional information that is not directly included in AS Biology. However it can help to consolidate other techniques.


1.      Complementary DNA   [back to top]

Complementary DNA (cDNA) is DNA made from mRNA. This makes use of the enzyme reverse transcriptase, which does the reverse of transcription: it synthesises DNA from an RNA template. It is produced naturally by a group of viruses called the retroviruses (which include HIV), and it helps them to invade cells. In genetic engineering reverse transcriptase is used to make an artificial gene of cDNA as shown in this diagram.

Complementary DNA has helped to solve different problems in genetic engineering:

It makes genes much easier to find. There are some 70 000 genes in the human genome, and finding one gene out of this many is a very difficult (though not impossible) task. However a given cell only expresses a few genes, so only makes a few different kinds of mRNA molecule. For example the b cells of the pancreas make insulin, so make lots of mRNA molecules coding for insulin. This mRNA can be isolated from these cells and used to make cDNA of the insulin gene.  

2.      Restriction Enzymes  [back to top]

These are enzymes that cut DNA at specific sites. They are properly called restriction endonucleases because they cut the bonds in the middle of the polynucleotide chain. Some restriction enzymes cut straight across both chains, forming blunt ends, but most enzymes make a staggered cut in the two strands, forming sticky ends.


The cut ends are “sticky” because they have short stretches of single-stranded DNA with complementary sequences. These sticky ends will stick (or anneal) to another piece of DNA by complementary base pairing, but only if they have both been cut with the same restriction enzyme. Restriction enzymes are highly specific, and will only cut DNA at specific base sequences,  4-8 base pairs long, called recognition sequences.

Restriction enzymes are produced naturally by bacteria as a defence against viruses (they “restrict” viral growth), but they are enormously useful in genetic engineering for cutting DNA at precise places ("molecular scissors"). Short lengths of DNA cut out by restriction enzymes are called restriction fragments. There are thousands of different restriction enzymes known, with over a hundred different recognition sequences. Restriction enzymes are named after the bacteria species they came from, so EcoR1 is from E. coli strain R, and HindIII is from Haemophilis influenzae.  

3.      DNA Ligase   [back to top]

This enzyme repairs broken DNA by joining two nucleotides in a DNA strand. It is commonly used in genetic engineering to do the reverse of a restriction enzyme, i.e. to join together complementary restriction fragments.

The sticky ends allow two complementary restriction fragments to anneal, but only by weak hydrogen bonds, which can quite easily be broken, say by gentle heating. The backbone is still incomplete.

DNA ligase completes the DNA backbone by forming covalent bonds. Restriction enzymes and DNA ligase can therefore be used together to join lengths of DNA from different sources.  

4.      Vectors  [back to top]

In biology a vector is something that carries things between species. For example the mosquito is a disease vector because it carries the malaria parasite into humans. In genetic engineering a vector is a length of DNA that carries the gene we want into a host cell. A vector is needed because a length of DNA containing a gene on its own won’t actually do anything inside a host cell. Since it is not part of the cell’s normal genome it won’t be replicated when the cell divides, it won’t be expressed, and in fact it will probably be broken down pretty quickly. A vector gets round these problems by having these properties:

Many different vectors have been made for different purposes in genetic engineering by modifying naturally-occurring DNA molecules, and these are now available off the shelf. For example a cloning vector contains sequences that cause the gene to be copied (perhaps many times) inside a cell, but not expressed. An expression vector contains sequences causing the gene to be expressed inside a cell, preferably in response to an external stimulus, such as a particular chemical in the medium. Different kinds of vector are also available for different lengths of DNA insert:

Type of vector

Max length of DNA insert


                 10 kbp

Virus or phage

                 30 kbp

Bacterial Artificial Chromosome (BAC)

               500 kbp


5.      Plasmids  [back to top]

Plasmids are by far the most common kind of vector, so we shall look at how they are used in some detail. Plasmids are short circular bits of DNA found naturally in bacterial cells. A typical plasmid contains 3-5 genes and there are usually around 10 copies of a plasmid in a bacterial cell. Plasmids are copied separately from the main bacterial DNA when the cell divides, so the plasmid genes are passed on to all daughter cells. They are also used naturally for exchange of genes between bacterial cells (the nearest they get to sex), so bacterial cells will readily take up a plasmid. Because they are so small, they are easy to handle in a test tube, and foreign genes can quite easily be incorporated into them using restriction enzymes and DNA ligase.

One of the most common plasmids used is the R-plasmid (or pBR322). This plasmid contains a replication origin, several recognition sequences for different restriction enzymes (with names like PstI and EcoRI), and two marker genes, which confer resistance to different antibiotics (ampicillin and tetracycline).  

The diagram below shows how DNA fragments can be incorporated into a plasmid using restriction and ligase enzymes. The restriction enzyme used here (PstI) cuts the plasmid in the middle of one of the marker genes (we’ll see why this is useful later). The foreign DNA anneals with the plasmid and is joined covalently by DNA ligase to form a hybrid vector (in other words a mixture or hybrid of bacterial and foreign DNA).  Several other products are also formed: some plasmids will simply re-anneal with themselves to re-form the original plasmid, and some DNA fragments will join together to form chains or circles. Theses different products cannot easily be separated, but it doesn’t matter, as the marker genes can be used later to identify the correct hybrid vector.

6.      Gene Transfer  [back to top]

Vectors containing the genes we want must be incorporated into living cells so that they can be replicated or expressed. The cells receiving the vector are called host cells, and once they have successfully incorporated the vector they are said to be transformed. Vectors are large molecules which do not readily cross cell membranes, so the membranes must be made permeable in some way. There are different ways of doing this depending on the type of host cell.

1.  Bacteriophages (or phages) are viruses that infect bacteria. They are a very effective way of delivering large genes into bacteria cells in culture.

2.  Adenoviruses are human viruses that causes respiratory diseases including the common cold. Their genetic material is double-stranded DNA, and they are ideal for delivering genes to living patients in gene therapy. Their DNA is not incorporated into the host’s chromosomes, so it is not replicated, but their genes are expressed.

The adenovirus is genetically altered so that its coat proteins are not synthesised, so new virus particles cannot be assembled and the host cell is not killed.

3.  Retroviruses are a group of human viruses that include HIV. They are enclosed in a lipid membrane and their genetic material is double-stranded RNA. On infection this RNA is copied to DNA and the DNA is incorporated into the host’s chromosome. This means that the foreign genes are replicated into every daughter cell.

After a certain time, the dormant DNA is switched on, and the genes are expressed in all the host cells.

7.      Genetic Markers  [back to top]

These are needed to identify cells that have successfully taken up a vector and so become transformed. With most of the techniques above less than 1% of the cells actually take up the vector, so a marker is needed to distinguish these cells from all the others. We’ll look at how to do this with bacterial host cells, as that’s the most common technique.

A common marker, used in the R-plasmid, is a gene for resistance to an antibiotic such as tetracycline. Bacterial cells taking up this plasmid can make this gene product and so are resistant to this antibiotic. So if the cells are grown on a medium containing tetracycline all the normal untransformed cells, together with cells that have taken up DNA that’s not in a plasmid (99%) will die. Only the 1% transformed cells will survive, and these can then be grown and cloned on another plate.

8.      Replica Plating  [back to top]

Replica plating is a simple technique for making an exact copy of an agar plate. A pad of sterile cloth the same size as the plate is pressed on the surface of an agar plate with bacteria growing on it. Some cells from each colony will stick to the cloth. If the cloth is then pressed onto a new agar plate, some cells will be deposited and colonies will grow in exactly the same positions on the new plate. This technique has a number of uses, but the most common use in genetic engineering is to help solve another problem in identifying transformed cells.

This problem is to distinguish those cells that have taken up a hybrid plasmid vector (with a foreign gene in it) from those cells that have taken up the normal plasmid. This is where the second marker gene (for resistance to ampicillin) is used. If the foreign gene is inserted into the middle of this marker gene, the marker gene is disrupted and won't make its proper gene product. So cells with the hybrid plasmid will be killed by ampicillin, while cells with the normal plasmid will be immune to ampicillin. Since this method of identification involves killing the cells we want, we must first make a master agar plate and then make a replica plate of this to test for ampicillin resistance.

Once the colonies of cells containing the correct hybrid plasmid vector have been identified, the appropriate colonies on the master plate can be selected and grown on another plate.

The R-plasmid with its antibiotic-resistance genes dates from the early days of genetic engineering in the 1970s. In recent years better plasmids with different marker genes have been developed that do not kill the desired cells, and so do not need a replica plate. These new marker genes make an enzyme (actually lactase) that converts a colourless substrate in the agar medium into a blue-coloured product that can easily be seen. So cells with a normal plasmid turn blue on the correct medium, while those with the hybrid plasmid can't make the enzyme and stay white. These white colonies can easily be identified and transferred to another plate. Another marker gene, transferred from jellyfish, makes a green fluorescent protein (GFP).

9.      Polymerase Chain Reaction (PCR)  [back to top]

Genes can be cloned by cloning the bacterial cells that contain them, but this requires quite a lot of DNA in the first place. PCR can clone (or amplify) DNA samples as small as a single molecule. It is a newer technique, having been developed in 1983 by Kary Mullis, for which discovery he won the Nobel prize in 1993. The polymerase chain reaction is simply DNA replication in a test tube. If a length of DNA is mixed with the four nucleotides (A, T, C and G) and the enzyme DNA polymerase in a test tube, then the DNA will be replicated many times. The details are shown in this diagram:


  1. Start with a sample of the DNA to be amplified, and add the four nucleotides and the enzyme DNA polymerase.

  2. Normally (in vivo) the DNA double helix would be separated by the enzyme helicase, but in PCR (in vitro) the strands are separated by heating to 95°C for two minutes. This breaks the hydrogen bonds.

  3. DNA polymerisation always requires short lengths of DNA (about 20 bp long) called primers, to get it started. In vivo the primers are made during replication by DNA polymerase, but in vitro they must be synthesised separately and added at this stage. This means that a short length of the sequence of the DNA must already be known, but it does have the advantage that only the part between the primer sequences is replicated. The DNA must be cooled to 40°C to allow the primers to anneal to their complementary sequences on the separated DNA strands.

  4. The DNA polymerase enzyme can now extend the primers and complete the replication of the rest of the DNA. The enzyme used in PCR is derived from the thermophilic bacterium Thermus aquaticus, which grows naturally in hot springs at a temperature of 90°C, so it is not denatured by the high temperatures in step 2. Its optimum temperature is about 72°C, so the mixture is heated to this temperature for a few minutes to allow replication to take place as quickly as possible.

  5. Each original DNA molecule has now been replicated to form two molecules. The cycle is repeated from step 2 and each time the number of DNA molecules doubles. This is why it is called a chain reaction, since the number of molecules increases exponentially, like an explosive chain reaction. Typically PCR is run for 20-30 cycles.

PCR can be completely automated, so in a few hours a tiny sample of DNA can be amplified millions of times with little effort. The product can be used for further studies, such as cloning, electrophoresis, or gene probes. Because PCR can use such small samples it can be used in forensic medicine (with DNA taken from samples of blood, hair or semen), and can even be used to copy DNA from mummified human bodies, extinct woolly mammoths, or from an insect that's been encased in amber since the Jurassic period. One problem of PCR is having a pure enough sample of DNA to start with. Any contaminant DNA will also be amplified, and this can cause problems, for example in court cases.  

10.    DNA Probes  [back to top]

These are used to identify and label DNA fragments that contain a specific sequence. A probe is simply a short length of DNA (20-100 nucleotides long) with a label attached. There are two common types of label used:

Probes are always single-stranded, and can be made of DNA or RNA. If a probe is added to a mixture of different pieces of DNA (e.g. restriction fragments) it will anneal (base pair) with any lengths of DNA containing the complementary sequence. These fragments will now be labelled and will stand out from the rest of the DNA. DNA probes have many uses in genetic engineering:

11.    Shotguning  [back to top]

This is used to find one particular gene in a whole genome, a bit like finding the proverbial needle in a haystack. It is called the shotgun technique because it starts by indiscriminately breaking up the genome (like firing a shotgun at a soft target) and then sorting through the debris for the particular gene we want. For this to work a gene probe for the gene is needed, which means at least a short part of the gene’s sequence must be known.  

12.   Antisense Genes  [back to top]

These are used to turn off the expression of a gene in a cell. The principle is very simple: a copy of the gene to be switch off is inserted into the host genome the “wrong” way round, so that the complementary (or antisense) strand is transcribed. The antisense mRNA produced will anneal to the normal sense mRNA forming double-stranded RNA. Ribosomes can’t bind to this, so the mRNA is not translated, and the gene is effectively “switched off”.

13.    Gene Synthesis  [back to top]

It is possible to chemically synthesise a gene in the lab by laboriously joining nucleotides together in the correct order.  Automated machines can now make this much easier, but only up to a limit of about 30bp, so very few real genes could be made this way (anyway it’s usually much easier to make cDNA). The genes for the two insulin chains (xx bp) and for the hormone somatostatin (42 bp) have been synthesisied this way. It is very useful for making gene probes.

14.    Electrophoresis  [back to top]

This is a form of chromatography used to separate different pieces of DNA on the basis of their length. It might typically be used to separate restriction fragments. The DNA samples are placed into wells at one end of a thin slab of gel made of agarose or polyacrylamide, and covered in a buffer solution. An electric current is passed through the gel. Each nucleotide in a molecule of DNA contains a negatively-charged phosphate group, so DNA is attracted to the anode (the positive electrode). The molecules have to diffuse through the gel, and smaller lengths of DNA move faster than larger lengths, which are retarded by the gel. So the smaller the length of the DNA molecule, the further down the gel it will move in a given time. At the end of the run the current is turned off.

Unfortunately the DNA on the gel cannot be seen, so it must be visualised. There are three common methods for doing this:


Applications of Genetic Engineering
[back to top]

This section contains additional information that is not directly included in AS Biology.  However it can be useful to help support and consolidate GE techniques.


We have now looked at some of the many techniques used by genetic engineers. What can be done with these techniques? By far the most numerous applications are still as research tools, and the techniques above are helping geneticists to understand complex genetic systems. Despite all the hype, genetic engineering still has very few successful commercial applications, although these are increasing each year. The applications so far can usefully be considered in three groups.

  • Gene Products

using genetically modified organisms (usually microbes) to produce chemicals, usually for medical or industrial applications.

  • New Phenotypes

using gene technology to alter the characteristics of organisms (usually farm animals or crops)

  • Gene Therapy

using gene technology on humans to treat a disease


Gene Products  [back to top]

The biggest and most successful kind of genetic engineering is the production of gene products. These products are of medical, agricultural or commercial value. This table shows a few of the examples of genetically engineered products that are already available.  



Host Organism


human hormone used to treat diabetes

bacteria /yeast


human growth hormone, used to treat dwarfism



bovine growth hormone, used to increase milk yield of cows


Factor VIII

human blood clotting factor, used to treat haemophiliacs



anti-blood clotting agent used in surgery



antibiotic, used to kill bacteria

fungi / bacteria


hepatitis B antigen, for vaccination



enzyme used to treat cystic fibrosis and emphysema



enzyme used to treat Pompe’s disease



enzyme used to treat CF



enzyme used in manufacture of cheese

bacteria /yeast


enzyme used in paper production



biodegradable plastic


The products are mostly proteins, which are produced directly when a gene is expressed, but they can also be non-protein products produced by genetically-engineered enzymes. The basic idea is to transfer a gene (often human) to another host organism (usually a microbe) so that it will make the gene product quickly, cheaply and ethically. It is also possible to make “designer proteins” by altering gene sequences, but while this is a useful research tool, there are no commercial applications yet.

Since the end-product is just a chemical, in principle any kind of organism could be used to produce it. By far the most common group of host organisms used to make gene products are the bacteria, since they can be grown quickly and the product can be purified from their cells. Unfortunately bacteria cannot not always make human proteins, and recently animals and even plants have also been used to make gene products. In neither case is it appropriate to extract the product from their cells, so in animals the product must be secreted in milk or urine, while in plants the product must be secreted from the roots. This table shows some of the advantages and disadvantages of using different organisms for the production of genetically-engineered gene products.  


Type of organism



Prokaryotes (Bacteria)

No nucleus so DNA easy to modify; have plasmids; small genome; genetics well understood; asexual so can be cloned; small and fast growing; easy to grow commercially in fermenters; will use cheap carbohydrate; few ethical problems.

Can’t splice introns; no post-translational modification; small gene size


Can splice introns; can do post-translational modifications; can accept large genes

Do not have plasmids (except yeast); often diploid so two copies of genes may need to be inserted; control of expression not well understood.

Fungi (yeast, mould)

Asexual so can be cloned; haploid, so only one copy needed; can be grown in vats

Can’t always make animals gene products


Photosynthetic so don’t need much feeding; can be cloned from single cells; products can be secreted from roots or in sap.

Cell walls difficult to penetrate by vector; slow growing; must be grown in fields; multicellular

Animals (pharming)

Most likely to be able to make human proteins; products can be secreted in milk or urine

Multicellular; slow growing

 We’ll look at some examples in detail  

Human Insulin  [back to top]

Insulin is a small protein hormone produced by the pancreas to regulate the blood sugar concentration. In the disease insulin-dependent diabetes the pancreas cells don’t produce enough insulin, causing wasting symptoms and eventually death. The disease can be successfully treated by injection of insulin extracted from the pancreases of slaughtered cows and pigs. However the insulin from these species has a slightly different amino acid sequence from human insulin and this can lead to immune rejection and side effects.

The human insulin gene was isolated, cloned and sequenced in the 1970s, and so it became possible to insert this gene into bacteria, who could then produce human insulin in large amounts. Unfortunately it wasn’t that simple. In humans, pancreatic cells first make pro-insulin, which then undergoes post-translational modification to make the final, functional insulin. Bacterial cells cannot do post-translational modification. Eventually a synthetic cDNA gene was made and inserted into the bacterium E. coli, which made pro-insulin, and the post-translational conversion to insulin was carried out chemically. This technique was developed by Eli Lilly and Company in 1982 and the product, “humulin” became the first genetically-engineered product approved for medical use.

In the 1990s the procedure was improved by using the yeast Saccharomyces cerevisiae instead of E. coli. Yeast, as a eukaryote, is capable of post-translational modification, so this simplifies the production of human insulin. However another company has developed a method of converting pig insulin into human insulin by chemically changing a few amino acids, and this turns out to be cheaper than the genetic engineering methods. This all goes to show that genetic engineers still have a lot to learn.

Human Growth Hormone (HGH)  [back to top]

HGH is a protein hormone secreted by the pituitary gland, which stimulates tissue growth. Low production of HGH in childhood results in pituitary dwarfism. This can be treated with HGH extracted from dead humans, but as the treatment caused some side effects, such as Creutzfeldt-Jacod disease (CJD), the treatment was withdrawn. The HGH gene has been cloned and an artificial cDNA gene has been inserted into E. coli. A signal sequence has been added which not only causes the gene to be translated but also causes the protein to be secreted from the cell, which makes purification much easier. This genetically engineered HGH is produced by Genentech and can successfully restore normal height to children with HGH defficiency.

Bovine Somatotrophin (BST)  [back to top]

This is a growth hormone produced by cattle. The gene has been cloned in bacteria by the company Monsanto, who can produce large quantities of BST. in the USA cattle are often injected with BST every 2 weeks, resulting in a 10% increase in mass in beef cattle and a 25% increase in milk production in dairy cows. BST was tested in the UK in 1985, but it was not approved and its use is currently banned in the EU. This is partly due to public concerns and partly because there is already overproduction of milk and beef in the EU, so greater production is not necessary.

Rennin  [back to top]

Rennin is an enzyme used in the production of cheese. It is produced in the stomach of juvenile mammals (including humans) and it helps the digestion of the milk protein caesin by solidifying it so that is remains longer in the stomach. Traditionally the cheese industry has used rennin obtained from the stomach of young calves when they are slaughtered for veal, but there are moral and practical objections to this source. Now an artificial cDNA gene for rennin has been made from mRNA extracted from calf stomach cells, and this gene has been inserted into a variety of microbes such as the bacterium E. coli and the fungus Aspergillus niger. The rennin extracted from these microbes has been very successful and 90% of all hard cheeses in the UK are made using microbial rennin. Sometimes (though not always) these products are labelled as “vegetarian cheese”.


AAT (a-1-antitrypsin)  [back to top]

AAT is a human protein made in the liver and found in the blood. As the name suggests it is an inhibitor of protease enzymes like trypsin and elastase. There is a rare mutation of the AAT gene (a single base substitution) that causes AAT to be inactive, and so the protease enzymes to be uninhibited. The most noticeable effect of this in the lungs, where elastase digests the elastic tissue of the alveoli, leading to the lung disease emphysema. This condition can be treated by inhaling an aerosol spray containing AAT so that it reaches the alveoli and inhibits the elastase there.

AAT for this treatment can be extracted from blood donations, but only in very small amounts. The gene for AAT has been found and cloned, but AAT cannot be produced in bacteria because AAT is glycoprotein, which means it needs to have sugars added by post translational modification. This kind of modification can only be carried out by animals, and AAT is now produced by genetically-modified sheep. In order to make the AAT easy to extract, the gene was coupled to a promoter for the milk protein b-lactoglubulin. Since this promoter is only activated in mammary gland cells, the AAT gene will only be expressed in mammary gland cells, and so will be secreted into the sheep's milk. This makes it very easy to harvest and purify without harming the sheep. The first transgenic sheep to produce AAT was called Tracy, and she was produced by PPL Pharmaceuticals in Edinburgh in 1993. This is how Tracy was made:

  1. A female sheep is given a fertility drug to stimulate her egg production, and several mature eggs are collected from her ovaries.

  1. The eggs are fertilised in vitro.

  1. A plasmid is prepared containing the gene for human AAT and the promoter sequence for b-lactoglobulin. Hundreds of copies of this plasmid are microinjected into the nucleus of the fertilised zygotes. Only a few of the zygotes will be transformed, but at this stage you can’t tell which.

  1. The zygotes divide in vitro until the embryos are at the 16-cell stage.

  1. The 16-cell embryos are implanted into the uterus of surrogate mother ewes. Only a few implantations result in a successful pregnancy.

  1. Test all the offspring from the surrogate mothers for AAT production in their milk. This is the only way to find if the zygote took up the AAT gene so that it can be expressed. About 1 in 20 eggs are successful.

  1. Collect milk from the transgenic sheep for the rest of their lives. Their milk contains about 35 g of AAT per litre of milk. Also breed from them in order to build up a herd of transgenic sheep.

  1. Purify the AAT, which is worth about £50 000 per mg.


New Phenotypes  [back to top]

This means altering the characteristics of organisms by genetic engineering. The organisms are generally commercially-important crops or farm animals, and the object is to improve their quality in some way. This can be seen as a high-tech version of selective breeding, which has been used by humans to alter and improve their crops and animals for at least 10 000 years. Nevertheless GMOs have turned out to be a highly controversial development. We don’t need to study any of these in detail, but this table gives an idea of what is being done.  



Long life tomatoes

There are two well-known projects, both affecting the gene for the enzyme  polygalactourinase (PG), a pectinase that softens fruits as they ripen. Tomatoes that make less PG ripen more slowly and retain more flavour. The American “Flavr Savr” tomato used antisense technology to silence the gene, while the British Zeneca tomato disrupted the gene. Both were successful and were on sale for a few years, but neither is produced any more.

Insect-resistant crops

Genes for various powerful protein toxins have been transferred from the bacterium Bacillus thuringiensis to crop plants including maize, rice and potatoes. These Bt toxins are thousands of times more powerful than chemical insecticides, and since they are built-in to the crops, insecticide spraying (which is non-specific and damages the environment) is unnecessary.

Virus-resistant crops

Gene for virus coat protein has been cloned and inserted into tobacco, potato and tomato plants. The coat protein seems to “immunise” the plants, which are much more resistant to viral attack.

Herbicide resistant crops

The gene for resistance to the herbicide BASTA has been transferred from Streptomyces bacteria to tomato, potato, corn, and wheat plants, making them resistant to Basta. Fields can safely be sprayed with this herbicide, which will kill all weeds, but the crops. However, this means using more agrochemicals, not less.

Pest-resistant legumes

The gene for an enzyme that synthesises a chemical toxic to weevils has been transferred from Bacillus bacteria to The Rhizobium bacteria that live in the root nodules of legume plants. These root nodules are now resistant to attack by weevils.

Nitrogen-fixing crops

This is a huge project, which aims to transfer the 15-or-so genes required for nitrogen fixation from the nitrogen-fixing bacteria Rhizobium into cereals and other crop plants. These crops would then be able to fix their own atmospheric nitrogen and would not need any fertiliser. However, the process is extremely complex, and the project is nowhere near success.

Crop improvement

Proteins in some crop plants, including wheat, are often deficient in essential amino acids (which is why vegetarians have to watch their diet so carefully), so the protein genes are being altered to improve their composition for human consumption.

Mastitis-resistant cattle

The gene for the enzyme lactoferrin, which helps to resists the infection that causes the udder disease mastitis, has been introduced to Herman – the first transgenic bull. Herman’s offspring inherit this gene, do not get mastitis and so produce more milk.

Tick-resistant sheep

The gene for the enzyme chitinase, which kills ticks by digesting their exoskeletons, has bee transferred from plants to sheep. These sheep should be immune to tick parasites, and may not need sheep dip.

Fast-growing sheep

The human growth hormone gene has been transferred to sheep, so that they produce human growth hormone and grow more quickly. However they are more prone to infection and the females are infertile.

Fast-growing fish

A number of fish species, including salmon, trout and carp, have been given a gene from another fish (the ocean pout) which activates the fish’s own growth hormone gene so that they grow larger and more quickly. Salmon grow to 30 times their normal mass at 10 times the normal rate.

Environment cleaning microbes

Genes for enzymes that digest many different hydrocarbons found in crude oil have been transferred to Pseudomonas bacteria so that they can clean up oil spills.


Gene Therapy  [back to top]

This is perhaps the most significant, and most controversial kind of genetic engineering. It is also the least well-developed. The idea of gene therapy is to genetically alter humans in order to treat a disease. This could represent the first opportunity to cure incurable diseases.  Note that this is quite different from using genetically-engineered microbes to produce a drug, vaccine or hormone to treat a disease by conventional means. Gene therapy means altering the genotype of a tissue or even a whole human.

Cystic Fibrosis  [back to top]

Cystic fibrosis (CF) is the most common genetic disease in the UK, affecting about 1 in 2500. It is caused by a mutation in the gene for protein called CFTR (Cystic Fibrosis Transmembrane Regulator). The gene is located on chromosome 7, and there are actually over 300 different mutations known, although the most common mutation is a deletion of three bases, removing one amino acid out of 1480 amino acids in the protein. CFTR is a chloride ion channel protein found in the cell membrane of epithelial (lining) tissue cells, and the mutation stops the protein working, so chloride ions cannot cross the cell membrane.

Chloride ions build up inside these cells, which cause sodium ions to enter to balance the charge, and the increased concentration of the both these ions inside the epithelial cells decreases the osmotic potential. Water is therefore retained inside the cells, which means that the mucus secreted by these cells is drier and more sticky than normal. This sticky mucus block the tubes into which it is secreted, such as the small intestine, pancreatic duct, bile duct, sperm duct, bronchioles and alveoli.

These blockages lead to the symptoms of CF: breathlessness, lung infections such as bronchitis and pneumonia, poor digestion and absorption, and infertility. Of these symptoms the lung effects are the most serious causing 95% of deaths. CF is always fatal, though life expectancy has increased from 1 year to about 20 years due to modern treatments. These treatments include physiotherapy many times each day to dislodge mucus from the lungs, antibiotics to fight infections, DNAse drugs to loosen the mucus, enzymes to help food digestion and even a heart-lung transplant.

Given these complicated (and ultimately unsuccessful) treatments, CF is a good candidate for gene therapy, and was one of the first diseases to be tackled this way. The gene for CFTR was identified in 1989 and a cDNA clone was made soon after. The idea is to deliver copies of this good gene to the epithelial cells of the lung, where they can be incorporated into the nuclear DNA and make functional CFTR chloride channels. If about 10% of the cells could be corrected, this would cure the disease.

Two methods of delivery are being tried: liposomes and adenoviruses, both delivered with an aerosol inhaler, like those used by asthmatics. Clinical trials are currently underway, but as yet no therapy has been shown to be successful.


SCID  [back to top]

Severe Combined Immunodefficiency Disease (SCID) is a rare genetic disease that affects the immune system. It is caused by a mutation in the gene for the enzyme adenosine deaminase (ADA). Without this enzyme white blood cells cannot be made, so sufferers have almost no effective immune system and would quickly contract a fatal infection unless they spend their lives in sterile isolation (SCID is also known as “baby in a bubble syndrome”). Gene therapy has been attempted with a few children in the USA and UK by surgically removing bone marrow cells (which manufacture white blood cells in the body) from the patient, transfecting them with a genetically-engineered virus containing the ADA gene, and then returning the transformed cells to the patient. The hope is that these transformed cells will multiply in the bone marrow and make white blood cells. The trials are still underway, so the success is unknown.

The Future of Gene Therapy 
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Gene therapy is in its infancy, and is still very much an area of research rather than application. No one has yet been cured by gene therapy, but the potential remains enticing. Gene therapy need not even be limited to treating genetic diseases, but could also help in treating infections and environmental diseases:

 Germ-line therapy would be highly effective, but is also potentially dangerous (since the long-term effects of genetic alterations are not known), unethical (since it could easily lead to eugenics) and immoral (since it could involve altering and destroying human embryos). It is currently illegal in the UK and most other countries, and current research is focusing on somatic cell therapy only. All gene therapy trials in the UK must be approved by the Gene Therapy Advisory Committee (GTAC), a government body that reviews the medical and ethical grounds for a trial. Germ-line modification is allowed with animals, and indeed is the basis for producing GMOs.




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Last updated 20/06/2004