Question
What are genetically modified (GM) crops? Explain the technology behind Bt cotton, golden rice, and Flavr Savr tomato with their specific gene modifications.
(NEET and CBSE 12 — a high-frequency topic with specific gene/organism matching)
Solution — Step by Step
Bt stands for Bacillus thuringiensis, a soil bacterium that produces a crystal protein (Cry protein) toxic to certain insects.
The cry gene (specifically cry1Ac and cry2Ab for cotton bollworm) is transferred from B. thuringiensis into cotton plants. The plant now produces the Cry protein in its tissues.
When the bollworm larva eats the Bt cotton leaf, the Cry protein is activated in the insect’s alkaline gut, binds to gut epithelial cells, creates pores, and kills the larva. The protein is harmless to humans (our gut is acidic).
Golden rice is engineered to produce beta-carotene (provitamin A) in the edible part (endosperm). Normal rice produces beta-carotene in leaves but not in grains.
Two genes were introduced:
- psy (phytoene synthase) from Narcissus pseudonarcissus (daffodil)
- crtI (carotene desaturase) from Erwinia uredovora (a bacterium)
These genes complete the beta-carotene biosynthetic pathway in rice endosperm. The golden colour comes from accumulated beta-carotene. Designed to address vitamin A deficiency in rice-dependent populations.
The Flavr Savr tomato (1994, Calgene) was the first commercially grown GM food crop. It was engineered for delayed ripening to improve shelf life.
The technique: an antisense RNA of the polygalacturonase (PG) gene was introduced. This antisense RNA binds to the normal PG mRNA and prevents its translation. PG enzyme (which degrades cell wall pectin, causing softening) is not produced, so the tomato stays firm longer.
graph TD
A[GM Crops] --> B["Bt Cotton"]
A --> C["Golden Rice"]
A --> D["Flavr Savr Tomato"]
B --> E["cry gene from B. thuringiensis"]
B --> F["Cry protein kills bollworm"]
C --> G["psy + crtI genes"]
C --> H["Beta-carotene in endosperm"]
D --> I["Antisense PG RNA"]
D --> J["Delayed ripening"]Why This Works
Each GM crop solves a specific agricultural problem using a targeted genetic modification:
- Bt cotton reduces pesticide use and crop loss from bollworm (saves Indian farmers crores annually)
- Golden rice addresses a nutritional deficiency in a staple food
- Flavr Savr reduces post-harvest loss by extending shelf life
The principle is the same in all cases: identify a gene that produces the desired trait, isolate it, and transfer it into the crop using recombinant DNA technology (usually via Agrobacterium tumefaciens or gene gun).
Alternative Method
For NEET, the critical matchings to memorise:
- Bt cotton: cry1Ac, cry2Ab genes — targets bollworm (lepidopteran insects)
- Bt corn: cry1Ab gene — targets corn borer
- Golden rice: psy gene (daffodil) + crtI gene (bacteria) — beta-carotene
The specific Cry protein numbers are tested. cry1 targets Lepidoptera (moths/butterflies), cry2 also targets Lepidoptera, cry3 targets Coleoptera (beetles).
Common Mistake
The most frequent error: saying “Bt cotton produces pesticide.” Bt cotton produces the Cry protein, which is an insecticidal protein, not a synthetic pesticide. The Cry protein is highly specific — it is toxic only to certain insect orders (those with alkaline guts) and is harmless to humans, birds, and beneficial insects. Calling it a “pesticide” is technically imprecise and can lead to wrong MCQ answers.
Also, students confuse the Cry protein (inactive crystalline form) with the active toxin. The Cry protein is a protoxin — it becomes active only when cleaved by proteases in the insect’s alkaline gut. This activation mechanism is why it is safe for mammals.