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Microarrays, bioinformatics and GMO risks and benefitsEdexcel International A Level Biology: Revision notes

Section 1

Microarrays: finding active genes

A microarray (gene chip) is a slide carrying thousands of tiny spots. Each spot contains many copies of single-stranded DNA from one known gene. It shows which genes are being transcribed in a tissue.

  1. Extract mRNA from the cells (it is present only for active genes).
  2. Make cDNA from the mRNA and label it with a fluorescent dye.
  3. Add the labelled cDNA to the microarray. It binds by complementary base pairing (hybridises) to the DNA at the matching spots.
  4. Wash off unbound cDNA and scan the slide. A fluorescent spot means that gene was active; brighter spots mean more mRNA.
Key termsmicroarraycDNAhybridisationfluorescent label
Common mistake

The array shows which genes are active (transcribed), not which genes are present. All cells of an organism carry the same genes in their DNA.

Section 2

Comparing two samples

Two samples, for example tumour and healthy tissue, can be labelled with different coloured dyes and added to the same microarray. Using red for tumour and green for healthy:

  • red spot: gene more active in the tumour
  • green spot: gene more active in the healthy tissue
  • yellow spot: active about equally in both
  • no colour: gene not active in either sample

Uses include finding genes involved in cancer, working out which treatment may suit a patient, and comparing a GM organism with its non-GM version to look for unintended changes.

Key termsdifferential expression

Section 3

Bioinformatics

Bioinformatics is the use of computer software, databases and statistics to store, analyse and compare biological data, such as DNA sequences, gene activity and protein sequences. Microarrays and genome sequencing produce huge amounts of data that cannot be analysed by hand.

Uses include:

  • finding genes in a newly sequenced genome
  • comparing sequences with databases of known genes to predict the function of an unknown gene, because genes with similar sequences often have similar functions
  • identifying genes for disease or for antibiotic resistance without lengthy laboratory tests
  • analysing which genes are active in microarray experiments
Key termsbioinformaticsdatabase

Section 4

Benefits of GMOs

  • Higher yields and less loss: crops with pest resistance (for example Bt toxin) or herbicide tolerance, so less insecticide is sprayed.
  • Improved nutrition: for example rice that makes β-carotene to reduce vitamin A deficiency.
  • Drugs and vaccines made by GM bacteria, plants and animals.
  • Crops that tolerate drought or salty soils, and longer shelf-life, which may reduce food waste.
Key termsherbicide tolerancepest resistance

Section 5

Risks of GMOs

  • Gene flow: pollen may transfer genes to wild relatives or non-GM crops, possibly creating herbicide-resistant weeds.
  • Resistance in pests: constant exposure to a toxin selects for resistant pests.
  • Non-target effects: toxins may harm harmless insects, and reduce biodiversity.
  • Health: possible allergies and unknown long-term effects, so GM foods need testing.
  • Economic and ethical issues: patents can make farmers dependent on companies; some people object to modifying organisms, especially animals.

When evaluating, give benefits and risks, use any data supplied (for example calculate a percentage change), and end with a justified conclusion.

Key termsgene flowbiodiversity
Exam tip

In an 'evaluate' question, a conclusion that the benefits do or do not outweigh the risks needs a reason that refers to the evidence in the question.

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Microarrays, bioinformatics and GMO risks and benefits

  1. A researcher wants to know which genes are active in a breast tumour compared with healthy breast tissue. mRNA is extracted from each tissue and used to make cDNA labelled with a fluorescent dye: red for the tumour sample and green for the healthy sample. The labelled samples are mixed and added to a microarray, a slide on which each spot contains many copies of single-stranded DNA from one known human gene. After washing, the colour of each spot is recorded.
    Explain why the researcher extracts mRNA, rather than DNA, from the tissues to find which genes are active.2 marks
  2. A laboratory sequences the whole genome of a newly discovered soil bacterium. The team uses computer software and online databases of known gene sequences to look for genes similar to known antibiotic-resistance genes.
    Suggest how bioinformatics could help the laboratory decide whether the bacterium may be resistant to antibiotics, and why this is quicker than testing the bacterium in the laboratory.2 marks
  3. In a three-year field trial a GM maize variety carrying a gene from Bacillus thuringiensis for a toxin that kills the caterpillars of the maize borer was compared with a non-GM variety. The mean yield of the GM maize was 11.2 t ha⁻¹ and of the non-GM maize 9.8 t ha⁻¹. Farmers growing the non-GM maize sprayed insecticide on average 3.1 times per season; those growing the GM maize sprayed on average 0.5 times per season.
    Explain how the use of the GM maize benefits farmers and the environment.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).