The main objective of biotechnology in the field of agriculture is to produce some such crops which are pest resistant and in which we can use pesticides to a minimum. So they are called genetically modified organisms GMO or GM crops, e.g., Bt - Crops.

Bt cotton plants are transgenic plants having the gene of Bacillus thuringiensis bacteria produce a toxic protein that kill insects of lepidopterons (tobacco, budworms) beetles, flies, moth etc. This toxin is actually a protein crystals which are formed in bacteria in certain phase of growth. Bacteria produced protein crystals are insoluble in bacteria and no longer harmful but when these crystals reached to the alkaline medium of larvae gut, crystals solubilised in epithelial cell and get activated. So that insecticidal protein kill the insect larvae.

Bt cotton is a genetically modified plant contain a gene (Cry IAc, Cry IIAb) cry genes of Bacillus thuringiensis (Bt). Which control lepidopteran insects by producing toxin protein. Normal cotton plant do not consists such genes. These cry gqnes which produced toxic protein crystals produce different strains of toxins. That cry gene isolated from bacteria and inserted into cotton plant DNA through rDNA technology where bacterial gene get cloned inside the cotton plant to produced large quantity of pro - toxic in plant cell. Specific cry gene are inserted to produce resistancy for particular insects according to the region of world.