Question
What are genetically modified (GM) crops? How were Bt cotton, golden rice, and Flavr Savr tomato developed, and what specific traits were engineered into each?
(NEET, CBSE 12 — GM crops appear in almost every NEET paper, especially Bt cotton and golden rice)
Solution — Step by Step
A genetically modified crop has had its DNA altered using recombinant DNA technology to introduce a gene from another organism. The foreign gene (transgene) gives the plant a new trait — pest resistance, nutritional enhancement, or longer shelf life.
The key distinction from traditional breeding: we are inserting a specific, well-characterised gene rather than shuffling thousands of genes through cross-pollination.
Source gene: cry genes (cry1Ac, cry2Ab) from the soil bacterium Bacillus thuringiensis.
Mechanism: The Bt gene produces Cry protein (crystal protein) inside the plant. When bollworm larvae feed on Bt cotton leaves, the Cry protein gets activated in the alkaline gut of the insect, binds to midgut epithelial cells, creates pores, and causes cell swelling and lysis — killing the larvae.
Why “cry” protein? It forms crystalline inclusions inside the bacterium. The inactive protoxin becomes active toxin only in insect gut (pH > 9.5), which is why it is safe for humans (our gut is acidic, pH ~2).
Source genes: psy gene from daffodil (Narcissus pseudonarcissus) and crt1 gene from the bacterium Erwinia uredovora.
Trait: Produces beta-carotene (provitamin A) in the rice endosperm, giving it a golden colour. Normal rice endosperm lacks this biosynthetic pathway entirely.
Purpose: Addresses Vitamin A deficiency (VAD), which causes blindness in ~500,000 children annually in developing countries. One serving of golden rice provides 60% of daily Vitamin A needs.
Technology: Antisense RNA technology. The gene for polygalacturonase (PG enzyme, which softens the fruit by degrading pectin in cell walls) was inserted in the reverse orientation.
Mechanism: The antisense RNA binds to the sense mRNA of PG, forming double-stranded RNA that cannot be translated. Result: PG enzyme production drops, pectin breakdown slows, and the tomato stays firm longer.
Historical note: This was the first commercially grown GM food (1994, by Calgene Inc., USA). It was later withdrawn due to high production costs, not safety issues.
flowchart TD
A["GM Crop Development"] --> B["Identify desired trait"]
B --> C["Isolate gene from source organism"]
C --> D["Insert into plant genome using Agrobacterium / gene gun"]
D --> E["Screen for successful transformants"]
E --> F["Field trials and safety testing"]
F --> G["Commercial release"]
H["Bt Cotton"] -->|"cry gene from B. thuringiensis"| I["Pest resistance"]
J["Golden Rice"] -->|"psy + crt1 genes"| K["Beta-carotene production"]
L["Flavr Savr"] -->|"Antisense PG gene"| M["Delayed ripening"]Why This Works
Each GM crop targets a single, well-defined biochemical pathway. Bt cotton works because the Cry protein has insect-specific toxicity — the alkaline gut activation is a natural safety switch. Golden rice works because the carotenoid biosynthesis pathway was simply missing from the endosperm, so adding two genes completed the pathway. Flavr Savr works because silencing one enzyme (PG) directly controls the rate-limiting step of fruit softening.
The common thread: precise single-gene or two-gene interventions that produce dramatic phenotypic changes. This is why these three examples are NCERT favourites — they beautifully illustrate how one gene can change one trait.
Alternative Approach — Comparison Table
| GM Crop | Gene Source | Trait | Technology |
|---|---|---|---|
| Bt cotton | Bacillus thuringiensis | Insect resistance | cry gene transfer |
| Golden rice | Daffodil + Erwinia | Provitamin A | Metabolic pathway engineering |
| Flavr Savr tomato | Same organism (tomato) | Delayed ripening | Antisense RNA |
Common Mistake
Students often write that Cry protein kills all insects. Wrong. It is specific to lepidopteran larvae (bollworms) for cry1Ac, and to coleopteran insects for cry3Bb. The protein is a protoxin that requires alkaline pH for activation — it does NOT affect beneficial insects, birds, or mammals. NEET has tested this specificity directly.
Remember the three GM crops by their purpose: Bt = Bug killer, Golden rice = Gold (vitamin A colour), Flavr Savr = Fresh longer. This mnemonic covers the most commonly tested facts.