Question

‘Cry’ gene is obtained from
a. Agrobacterium tumefaciens
b. Bacillus thuringiensis
c. Rhizobium leguminosarum
d. Rhizobium phaseoli

Answer 1

Hint: Genetically modified crops are the crops that contain and express a transgene. It is done through the mechanism of recombinant DNA-technology which involves introducing the desired gene from foreign sources to a particular organism.

Complete answer:
Bt toxin genes were isolated from the Bacillus thuringiensis and incorporated into several crop plants such as cotton. The choice of the genes depends upon the crop and targeted pest as most Bt toxins are insect group specific. The toxin is coded by the genes named Cry. The proteins encoded by the genes Cry IAc and CryIIAb control the cotton bollworms and that of CryIAb controls corn borer.

The toxin crystals are in an inactive stage and when an insect ingest these crystals, their alkaline digestive tracts make the crystals active and soluble. It is then liberated after the action of proteases on it. The toxin when absorbed in the gut cell membrane of the insect, paralyzes the digestive tract and creates a pore in it. The insects then stop eating and die.

Agrobacterium tumefaciens is used for the genetic transformation in plants. It is based on the Ti (Tumour inducing) plasmid which occurs in the soil bacterium, Agrobacterium tumefaciens .The bacterium is called natural genetic engineering because the genes carried by its plasmid affects the several plant parts.

Hence, the correct answer is option (B).

Additional information:
The toxin introduced from the cry gene have specific activities against insect species of the following orders:
- Diptera: It includes flies and mosquitoes.
- Coleoptera: It includes beetles.
- Lepidoptera: It includes moths and butterflies.
- Nematodes
- Hymenoptera: It includes wasps, bees, ants and sawflies.

Note: Bacillus thuringiensis is a biological pesticide to protect crops that contains cry genes in its plasmid DNA and it is not located in the chromosomal DNA in most strains. Cry genes code a toxin which is poisonous to some insects which are resistant to the plants.