Biotechnology and genetic modification

Cambridge IGCSE Biology 0610 · Core and Extended

Syllabus points

• Explain why bacteria are useful in biotechnology and genetic modification.
• Describe the use of yeast in bread making and in biofuel production.
• Describe the use of enzymes in biological washing powders and in the food industry.
• Describe the use of a fermenter to grow microorganisms.
• Explain genetic modification and outline the process using insulin as an example.
• Discuss the advantages and disadvantages of genetically modifying crops.

Why bacteria are so useful

This is a standard question and the answer is a list of properties, each with a reason:

• They reproduce very rapidly, so large quantities are produced quickly.
• They have few ethical concerns compared with using animals.
• They can be grown on inexpensive materials, including waste products.
• They possess plasmids, small rings of DNA that can be cut open and have genes inserted, which makes them easy to modify.
• The genetic code is universal, so a gene taken from a human works perfectly well inside a bacterium.

That last point is the one most often missed and the one that makes genetic modification possible at all: a gene means the same thing in every organism.

Yeast

Yeast is a single-celled fungus that respires anaerobically when oxygen is short:

glucose → alcohol + carbon dioxide

Bread making uses the carbon dioxide. Bubbles of gas are trapped in the dough and make it rise. The alcohol evaporates during baking.

Biofuel uses the alcohol. Yeast ferments sugar from crops such as sugar cane or maize, and the ethanol produced is used as fuel, on its own or mixed with petrol.

The same reaction, and which product is wanted decides the process. Bread making is aerobic at first and turns anaerobic; brewing is kept anaerobic throughout.

Enzymes in industry

Biological washing powders contain enzymes that break down stains into soluble products that wash away:

• Protease for blood, egg and other protein stains.
• Lipase for fat and grease.
• Amylase for starchy food stains.

Because the enzymes do the work, the wash can be at a lower temperature, which saves energy. The limitation is that the water must not be too hot or the enzymes denature, and that the enzymes can damage protein fibres such as wool and silk.

In the food industry:

• Pectinase breaks down pectin in fruit cell walls, increasing the yield of juice and making it clearer.
• Lactase breaks down lactose to glucose and galactose, producing lactose-free milk for people who cannot digest lactose.

Fermenters

A fermenter is a large vessel used to grow microorganisms in controlled conditions, for products such as penicillin, insulin and mycoprotein.

The conditions controlled, and why each matters:

• Temperature — kept at the optimum by a water jacket. The microorganisms release heat as they respire, so the jacket usually cools rather than heats.
• pH — monitored and adjusted, since enzymes work over a narrow range.
• Oxygen — supplied by sterile air, for aerobic respiration.
• Nutrients — a supply of food such as glucose.
• Stirring — paddles keep the microorganisms suspended and distribute heat, oxygen and nutrients evenly.
• Sterility — everything is sterilised first, so no other organism competes or contaminates the product.

Genetic modification

Genetic modification is changing the genetic material of an organism by removing, changing or inserting individual genes.

The process, using human insulin as the example the syllabus expects:

1. The human insulin gene is cut out of human DNA using restriction enzymes. These cut at specific sequences and leave short single-stranded ends called sticky ends.
2. A plasmid is removed from a bacterium and cut open with the same restriction enzyme, so its sticky ends are complementary to the gene's.
3. The gene and the plasmid are joined using the enzyme ligase, forming a recombinant plasmid.
4. The plasmid is inserted into a bacterium.
5. The bacteria are grown in a fermenter, where they multiply and produce human insulin, which is then extracted and purified.

Step 2 is the one worth being precise about. The same restriction enzyme must be used on both, because that is what makes the sticky ends complementary so the pieces fit together.

Bacterial insulin has real advantages over the animal insulin it replaced: it is identical to human insulin so it works better and causes fewer reactions, it can be produced in large quantities cheaply, and it avoids objections from people who do not wish to use animal products.

Genetically modified crops

Examples include crops made resistant to herbicides, so a field can be sprayed and only the weeds die; crops made resistant to insect pests, so less insecticide is needed; and crops with improved nutritional value, such as rice engineered to contain beta-carotene.

Advantages: higher yields, less pesticide use, better nutrition, crops that tolerate drought or poor soil, and longer shelf life.

Disadvantages: the modified gene may spread to wild relatives, for instance creating herbicide-resistant weeds; effects on other species and on human health over the long term are not fully known; seed is often expensive and controlled by a few companies; and reliance on a single modified variety reduces biodiversity.

A question asking for a disadvantage of herbicide resistance is usually looking for gene transfer to weeds, or for the increased herbicide use that resistance permits, rather than for a general worry.