CIE A Level Biology (9700) 2019-2021

Revision Notes

19.3.3 Production of Genetically Modified Crops

Production of GM Crops

  • Crop production can be enhanced by genetically modifying plants for resistance to herbicides and insecticides
  • Crops such as maize, cotton, tobacco and oil seed rape can also be modified in this way, increasing yields
  • The benefits of using genetic engineering rather than the more traditional selective breeding techniques to modify crops are:
    • Organisms with the desired characteristics are produced more quickly
    • All organisms will contain the desired characteristic (there is no chance that recessive alleles may arise in the population)
    • The desired characteristic may come from a different species / kingdom
    • Decrease in use of chemicals (pesticides or herbicides) – which may benefit other organisms
    • Less risk of harm to humans – spray drift or pesticide residue on food
  • There are disadvantages of using genetically engineered organisms to increase the productivity of the crop are (although research is ongoing):
    • The development of resistance for the genes that have been introduced
    • The risk of the gene spreading to wild relatives
    • The modified organism may become a pest
    • The reduction in biodiversity
    • Potential ecological effects (eg. harm to non-targeted species like the Monarch butterflies)
    • Possible risk to human health as an allergy (there are no long-term studies on the effect on human health)

Insect resistance

  • If crop plants can be made insect-resistant then that increases productivity as insects can affect areas of the plant required for growth by eating the:
    • Leaves (reducing the rate of photosynthesis)
    • Seeds (this is often the part that is sold by the farmer)
    • Fruit (prevents the development of seed and / or it might be the part being sold)
  • Maize and cotton have been genetically modified with a gene for Bt toxin which allows the plants to produce their own insecticide. The insects are killed after they ingest plant parts
  • The productivity of the genetically engineered insect-resistant maize and cotton plants increases (although insects have developed resistance to the Bt toxin genes). Another advantage of growing Bt maize and cotton is that less pesticides are used which could have ecological benefits (eg. non-targeted invertebrates not harmed) and could mean less risk for humans from spray drift and / or from pesticide residue on foods consumed
  • Tobacco plants have been modified with the Bt toxin gene to be resistant to budworm, however it is not been grown for commercial purposes

Herbicide resistance

  • Growing herbicide-resistant crops allows farmers to spray a herbicide on the crop after germination to kill the weeds that would compete with the growing crop for light, water and minerals. The herbicide-resistant crops therefore will be more productive, the farmer will have a higher yield
  • The resistant gene came from a strain of the bacterium Agrobacterium. This gene allows an enzyme in the crop to continue to synthesise three amino acids (phenylalanine, tyrosine and tryptophan) needed to produce proteins required in the growing tips of plants
  • The herbicide glyphosate (the active ingredient in Roundup™) inhibits the enzyme in plants without the resistant gene. Without the proteins being synthesised the plants die
  • The crops that have been genetically modified to be herbicide-resistant are:
    • Canola (oil seed rape)
    • Cotton
    • Maize

Exam Tip

Know the reasons why canola, cotton and maize have been genetically engineered and the advantages and disadvantages of growing these GM crops.

Author: Catherine

Cate has over 20 years’ experience teaching Biology to IGCSE, IB and A-level students in seven different countries across Asia, Europe, North America and the Middle East. This has given her a fine appreciation of different cultures, places and teaching methods. Cate has a keen interest in producing Biology revision resources that will help students engage with the subject.
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