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Spec Mapping — OCR H420 Module 6.1.3 — Manipulating genomes, content statements covering the production and use of genetically modified bacteria, plants and animals, together with the social, moral and ethical implications of GM technology (refer to the official OCR H420 specification document for exact wording). This lesson builds directly on the cloning workflow from Lesson 2 and supplies the ethical and applied context for the gene-therapy lesson that follows.
A genetically modified organism (GMO) — also called a transgenic organism — is one whose genome has been deliberately altered by genetic engineering. OCR A-Level Biology A specification 6.1.3 requires you to understand the applications of GM bacteria, plants and animals, to know named examples, and to weigh the ethical, economic and environmental arguments surrounding their use.
This is one of the most contested topics in A-Level Biology, and OCR examiners reward balanced, evidence-based discussion over polemic. The scientific consensus, summarised by bodies including the World Health Organization, the European Food Safety Authority, the Royal Society and the US National Academies, is that GM foods currently on the market are as safe to consume as their conventional counterparts under the substantial equivalence framework. Debate over GMOs is therefore typically less about acute food safety and more about economic models (farmer dependence on multinational seed suppliers, patent law, terminator technology), environmental risks (gene flow to wild relatives, evolution of resistant pests and weeds), and political-cultural questions (consumer choice, labelling, indigenous-knowledge sovereignty). At A* level you are expected to navigate these distinctions cleanly and avoid the rhetorical trap of treating "GMOs" as a single monolithic category.
Key Definitions:
- GMO — an organism whose DNA has been altered by genetic engineering, typically by inserting a gene from another species.
- Transgenic — containing DNA from a different species.
- Cisgenic — genetically modified using genes from the same species or a sexually compatible relative.
Bacteria were the first organisms to be genetically modified, and they remain the most widely used. They reproduce rapidly, are easy to transform, and secrete proteins directly into the growth medium.
Before 1982, insulin for diabetics was extracted from pig or cow pancreases — a laborious process that sometimes caused immune reactions because animal insulin differs slightly from human insulin. Genentech scientists cloned human insulin cDNA into E. coli (Lesson 2), producing identical human insulin in unlimited quantities. Today virtually all insulin used worldwide is recombinant.
GM bacteria also produce:
The advantages over extraction from animals include unlimited supply, purity, absence of pathogens and — critically — the ability to produce human proteins that animal sources cannot provide.
The global biopharmaceutical market — overwhelmingly dominated by recombinant proteins — is now several hundred billion dollars per year and growing faster than the small-molecule pharmaceutical market. Monoclonal antibody drugs (Humira, Keytruda, Avastin and others), produced in CHO cell lines transformed by exactly the techniques outlined in Lesson 2, individually generate annual revenues in the tens of billions. The economic case for the cloning workflow is therefore overwhelming.
Plants are harder to modify than bacteria because they have rigid cell walls, and genes must reach the nucleus of totipotent cells that can regenerate into a whole plant. The standard method uses Agrobacterium tumefaciens, a soil bacterium that naturally inserts part of its Ti plasmid into plant cells (causing crown gall disease). Scientists replace the tumour-causing genes with a gene of interest, and Agrobacterium does the insertion work for them.
flowchart TD
A[Gene of interest cloned into disarmed Ti plasmid] --> B[Transform Agrobacterium tumefaciens]
B --> C[Co-cultivate Agrobacterium with plant leaf discs]
C --> D[Agrobacterium T-DNA region inserts into plant nuclear DNA]
D --> E[Tissue culture: callus on hormone-rich medium]
E --> F[Plantlets regenerate on different hormone medium]
F --> G[Select transformants on antibiotic e.g. kanamycin]
G --> H[Screen for gene expression: GUS / GFP / Southern blot]
H --> I[Transgenic plant for greenhouse trial]
Alternatives include:
Golden Rice contains genes for the synthesis of β-carotene (a precursor of vitamin A). Two genes — psy from daffodil (later maize) and crtI from the bacterium Erwinia — redirect carotenoid biosynthesis so that the endosperm accumulates β-carotene, giving the rice its golden colour.
The aim was humanitarian: hundreds of millions of children in Asia suffer vitamin A deficiency, which causes blindness and increases mortality from measles and diarrhoea. A bowl of Golden Rice provides a significant fraction of the daily requirement. Despite being developed in 1999, regulatory and political opposition delayed its approval for cultivation until 2021 (Philippines).
Bt crops carry a gene from Bacillus thuringiensis encoding a protein (Bt toxin, formally a Cry protein) that is lethal to certain insect larvae but harmless to mammals. B. thuringiensis is a soil bacterium that has been used as a sprayed biopesticide by organic farmers since the 1930s; the engineered version of the same toxin inside the plant tissue, expressed by every cell, gives continuous in-planta protection rather than dependence on repeated spraying. Bt cotton, maize and brinjal are planted on tens of millions of hectares worldwide. They reduce insecticide use — Indian Bt cotton farmers cut pesticide applications by 40–80% — but raise concerns about resistance evolution in target pests and the effects on non-target insects such as monarch butterflies.
The molecular biology is highly target-specific. Cry proteins are activated by alkaline pH in insect midgut lumen, cleaved by insect proteases, and then bind to specific cadherin receptors on the gut epithelial cells of susceptible larvae — pore formation in the brush-border membrane follows, leading to gut lysis and death. Mammalian stomachs are acidic, lack the relevant cadherin receptors, and digest the Cry protein into harmless peptides. This receptor-specificity is the basis for the regulatory finding that Bt crops pose negligible direct risk to humans and most non-target invertebrates.
To manage resistance evolution, farmers planting Bt cotton in the United States and India are required to plant a refuge — a fraction (typically 20%) of the field area sown with non-Bt cotton — so that susceptible insects can survive, mate with resistant survivors of the Bt field, and dilute resistance alleles in the population. This is a textbook real-world application of population genetics, and OCR examiners can ask you to explain the rationale in evolutionary terms.
Monsanto's Roundup Ready soybeans, maize and cotton carry a gene for a glyphosate-resistant form of EPSP synthase (5-enolpyruvylshikimate-3-phosphate synthase). EPSP synthase is an enzyme in the shikimate pathway, which produces aromatic amino acids in plants and many microorganisms but is absent in animals — explaining glyphosate's relatively low toxicity to humans. Glyphosate normally competes with the natural substrate PEP at the EPSP synthase active site, blocking aromatic amino acid biosynthesis and killing the plant. The bacterial variant of EPSP synthase used in Roundup Ready crops binds substrate normally but does not bind glyphosate, so the engineered crop survives field-applied herbicide.
Farmers can spray glyphosate (trade name Roundup) to kill weeds without harming the crop. This has simplified farming and reduced tillage (with associated soil-erosion and carbon-sequestration benefits) but has driven the evolution of glyphosate-resistant "superweeds" such as Amaranthus palmeri (Palmer amaranth) and Conyza canadensis (horseweed). Resistance can arise via target-site mutation, gene amplification, or metabolic detoxification — multiple parallel pathways that population biology predicts whenever a single selective agent is applied broadly across many hectares.
The first GM food approved for sale (1994) was the Flavr Savr tomato, engineered with an antisense copy of the polygalacturonase gene to slow fruit softening. It flopped commercially but paved the way for later GM foods.
Animals are the hardest to modify, partly because of ethical constraints and partly because transformation must happen in the fertilised egg to affect the whole organism.
AquAdvantage salmon contain a growth hormone gene from Chinook salmon driven by a promoter from the ocean pout. The Chinook growth-hormone gene gives a more potent peptide than the Atlantic salmon's own gene; the ocean-pout antifreeze promoter is constitutively active year-round, whereas the Atlantic salmon's native growth-hormone promoter is downregulated in winter. The combined effect is that AquAdvantage fish grow continuously rather than seasonally and reach market size in roughly half the normal time. They are reared exclusively in inland tanks rather than sea cages, are female and triploid (sterile), and are confined to a single facility — multiple physical and biological containment barriers against escape into wild Atlantic salmon populations. Approved by the US FDA in 2015 and Health Canada in 2016, they are the first GM animal sold for food. The 25-year regulatory journey from AquaBounty Technologies' first 1989 application to 2015 approval illustrates the cautious pace of GM-animal authorisation compared with GM crops.
Knockout mice have a specific gene disabled. They are indispensable in medical research for studying gene function and modelling human disease (e.g. cystic fibrosis, Alzheimer's, cancer). The classical method, developed by Mario Capecchi, Martin Evans and Oliver Smithies (2007 Nobel Prize in Physiology or Medicine), uses homologous recombination in embryonic stem cells: a targeting construct disables the gene of interest, transfected ES cells with the disruption are selected, the cells are injected into a blastocyst, and chimeric founders are bred to homozygous knockouts. Modern CRISPR-Cas9 editing (covered in Lesson 4) accelerates this process from years to weeks. Conditional knockouts using Cre-loxP allow tissue-specific or temporally inducible gene inactivation, enabling researchers to disable a gene only in, say, hepatocytes from the age of six weeks, bypassing embryonic-lethal phenotypes.
Pharming is the use of GM animals as living bioreactors. The gene for a pharmaceutical protein is linked to a milk gene promoter (typically the β-casein or β-lactoglobulin promoter, both highly active in mammary epithelium) so the protein is secreted in the animal's milk, from which it is easily purified. ATryn, human antithrombin III extracted from milk of GM goats by GTC Biotherapeutics (now LFB Biotechnologies), was the first pharmed drug approved (2006 by EMA in the EU; 2009 by FDA in the US). It is used to prevent thrombosis in patients with hereditary antithrombin deficiency.
The principal advantage of pharming over microbial production is that mammary epithelial cells perform mammalian post-translational modifications — including correct glycosylation, disulphide bond formation, and proteolytic processing — that prokaryotes cannot. For complex glycoproteins (antibodies, clotting factors, antithrombin, alpha-1-antitrypsin) bacterial production yields inactive or immunogenic protein. Pharming offers a route around this constraint. Disadvantages include long lead times (years to establish a transgenic herd), animal-welfare considerations, and the regulatory burden of demonstrating consistency across milking sessions and across individual animals.
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