Genetic manipulationOxford AQA IGCSE Biology: Revision notes

Tissue culture is a cloning technique that uses small groups of cells taken from part of a plant. These cells are grown on a nutrient medium under sterile conditions, and they develop into new, genetically identical plants. Because large numbers of plants can be grown from a very small amount of original plant material, tissue culture is useful for producing many identical copies of a plant quickly, including rare or commercially valuable varieties.

Cuttings are an older, simpler cloning method traditionally used by gardeners. A section of a parent plant (such as a stem with a few leaves) is cut off and placed in soil or water, where it grows roots and develops into a new, separate plant. Because the cutting comes directly from the parent plant, it is genetically identical to it.

Both methods produce clones: genetically identical plants with no genetic variation between the copies and the parent plant.

Embryo transplants involve splitting apart the cells of a developing animal embryo at an early stage, before the cells have become specialised. Each split-off group of cells can develop into a separate, genetically identical embryo. These identical embryos are then transplanted into host mothers, where they develop into genetically identical offspring.

Adult cell cloning is a more involved technique:

1. The nucleus is removed from an unfertilised egg cell.
2. The nucleus from an adult body cell (for example, a skin cell) is inserted into the emptied egg cell.
3. An electric shock is used as a catalyst to make the egg cell begin to divide, forming embryo cells.
4. These embryo cells contain the same genetic information as the adult body cell that provided the nucleus, so they are genetically identical to the adult donor.
5. Once the embryo has developed into a ball of cells, it is inserted into the womb of an adult female to continue its development.

The resulting offspring is a clone of the animal that provided the body cell nucleus, not of the animal that provided the egg cell or the host mother.

Genetic engineering allows a gene to be transferred from the chromosomes of one organism (which could be a human or any other organism) into the cells of a different organism, giving that organism a new characteristic. The process involves three main steps:

1. Isolating the gene — enzymes are used to cut out the required gene from the chromosome of the donor organism.
2. Inserting the gene into a vector — the isolated gene is inserted into a vector, which is usually a bacterial plasmid or a virus.
3. Inserting the vector into the required cells — the vector carries the gene into the cells of the organism to be modified, where it becomes part of that organism's genetic material.

Genes can be transferred into the cells of animals, plants or microorganisms at an early stage of development, so that as the organism grows, it develops with the desired new characteristic built in from the start.

Crops that have had their genes modified using genetic engineering are called genetically modified (GM) crops. Common examples include crops that have been given genes making them resistant to attack by insects, or resistant to herbicides (chemicals used to kill weeds).

GM crops generally show increased yields compared with non-modified crops. This can happen because:

• Insect-resistant crops lose less of their harvest to pest damage
• Herbicide-resistant crops allow farmers to spray herbicide to kill weeds without harming the crop itself, so the crop faces less competition for light, water and nutrients

Because of these increased yields, GM crops are seen by many as a way to help produce more food from the same amount of farmland.

Alongside their benefits, GM crops have raised a number of concerns that need to be weighed against the benefits:

• Effects on wild flower and insect populations — genes for herbicide resistance or insect resistance could spread beyond the crop (for example, through cross-pollination with wild relatives), or the reduced presence of weeds and pest insects around GM crops could reduce food sources for wild flowers and insects, altering local ecosystems.
• Uncertainty about human health effects — there is uncertainty about the long-term effects on human health of eating GM crops, since they have not been part of the human diet for as long as non-modified crops.

Students should weigh up both the benefits (such as increased yields) and these concerns when making an informed judgement about the use of GM crops — the specification asks for balanced evaluation, not a fixed conclusion for or against.