6.8 - Genetic Engineering

What genetic engineering and GM organisms are

• Genetic engineering (also called genetic modification) is the transfer of a gene for a desirable characteristic from one organism's genome into another organism, so the second organism develops that characteristic.
• The genome is the complete set of genetic material in an organism.
• Genetic engineering changes the genome by removing, changing, or inserting individual genes - most often by adding a gene taken from a different organism.

GM and transgenic organisms

• An organism changed in this way is described as genetically modified (GM).
• Because the gene often comes from a different species, it's also called a transgenic organism - one that contains genes brought in from another type of organism.
• You can have transgenic bacteria, animals, and plants.
• Sometimes the gene is transferred while the organism is at an early stage of development (such as an egg or embryo), so the whole organism grows with the new characteristic in all its cells.

How genetic engineering is used

• Bacteria - Modified to make human insulin to treat diabetes
• Crops - Modified to resist insects, herbicides, or disease, or to improve the size and quality of their fruit
• Sheep - Modified to make useful substances such as drugs in their milk

The genetic-engineering process

• Genetic engineering moves a gene from one organism into another through a sequence of enzyme steps, ending with a step that picks out the cells that worked.
• Two key enzymes are used:
  1. Restriction enzyme - Recognises a specific base sequence and cuts the DNA, leaving short tails of unpaired bases called sticky ends
  2. Ligase - Joins sticky ends together

The steps

1. The required gene is cut out of the donor DNA with a restriction enzyme, leaving sticky ends.
2. A plasmid is cut open with the same restriction enzyme, giving it complementary sticky ends.
3. The gene and the plasmid are mixed with ligase, which joins their sticky ends to form a recombinant plasmid.
4. The recombinant plasmid acts as a vector and is put into host cells, such as bacteria.
5. The successfully modified cells are selected (not all host cells take up the new DNA).

Choice of vector

• The recombinant plasmid is the usual vector.
• A virus can be used as the vector instead.
• For modifying plants, the soil bacterium Agrobacterium tumefaciens - which naturally inserts its genes into plant DNA - can carry the gene in.

Selecting the modified cells

• Not all host cells take up the new DNA, so the ones that did must be picked out.
• A marker gene that codes for antibiotic resistance is inserted alongside the desired gene.
• The host bacteria are grown on a plate containing the antibiotic.
• Only the cells that took up the marker - and therefore the desired gene - survive and reproduce; the rest are killed.

GM crops - Herbicide and insect (Bt) resistance

Crops can be genetically modified (GM) to protect their yield by giving them resistance to herbicides or to insect pests.

Herbicide resistance

• A herbicide-resistant crop carries a gene that lets it survive a particular weedkiller (herbicide).
• Farmers can then spray the whole field with that herbicide to kill the weeds competing with the crop, without harming the crop itself.
• Example: one type of soya has been modified to resist the herbicide glyphosate.

Insect (Bt) resistance

• An insect-resistant crop carries a gene that protects it from the insects that would normally eat it.
• A common example uses the Bt gene from the bacterium Bacillus thuringiensis, which makes a toxin harmful to certain insect pests.
• Crops such as maize and cotton carrying this gene are protected from insect larvae.
• Because the crop defends itself, fewer plants are eaten, so the crop is protected and the yield is higher.

GM crops - Improved nutrition with golden rice

• Crops can be genetically modified to improve their nutritional value.
• The best-known example is golden rice.

Why golden rice was developed

• In some parts of the world, people rely heavily on rice, which contains very little vitamin A.
• A lack of vitamin A can cause blindness.

How golden rice works

1. Golden rice has been engineered to make beta-carotene - the orange substance also found in carrots - by adding a gene that controls beta-carotene production.
2. When the golden rice is eaten, the body converts the beta-carotene into vitamin A.
3. This helps to prevent vitamin A deficiency in populations whose diets would otherwise be short of it.

GM crops - Disease and virus resistance

• Crops can be genetically modified to resist diseases, including those caused by viruses.
• A gene that gives resistance is inserted into the crop, so the plants aren't damaged by the pathogen (disease-causing organism). This protects the harvest.
• Example: plum trees have been genetically modified to resist the plum pox virus, a disease that would otherwise damage the fruit.

Benefits for farmers

• Farmers lose less of their harvest to infection, so more food is produced.
• They may also need fewer chemical treatments to control the disease.

Gene therapy and gene technology in medicine

• Gene technology can be used in medicine to treat diseases caused by faulty genes.
• In gene therapy, a working copy of a gene is inserted into the cells of a person with an inherited disorder.
• Those cells can then carry out the function the faulty gene could not.
• This offers the promise of treating inherited disorders directly, rather than only managing their symptoms.

Practical and ethical concerns

• Stress and anxiety - Knowing from an early age that you carry genes making you susceptible to a serious disease can cause considerable stress, even if the disease never develops.
• Pressure not to have children - People found to carry "problem" genes could be pressured not to have children.
• Discrimination - Employers and insurers could discriminate against people who are genetically more likely to develop a disease, for example by refusing life insurance or jobs.
• Cost - Drugs tailored to particular people are expensive to develop, which could lead to a two-tier health service in which only wealthier people can afford the newest treatments.
• Uncertainty - It is unclear whether changing genes might do more harm than good, including unknown effects on future generations.