Give information about $Bt$ cotton.

(N/A) Some strains of Bacillus thuringiensis produce proteins that kill certain insects such as lepidopterans (tobacco budworm,armyworm),coleopterans (beetles) and dipterans (flies,mosquitoes).
$B$. thuringiensis forms protein crystals during a particular phase of their growth. These crystals contain a toxic insecticidal protein.
The $Bt$ toxin protein exists as an inactive protoxin,but once an insect ingests the inactive toxin,it is converted into an active form of toxin due to the alkaline $pH$ of the gut,which solubilizes the crystals. The activated toxin binds to the surface of midgut epithelial cells and creates pores that cause cell swelling and lysis,eventually causing the death of the insect.
Specific $Bt$ toxin genes were isolated from Bacillus thuringiensis and incorporated into several crop plants,such as cotton.
The choice of genes depends upon the crop and the targeted pest,as most $Bt$ toxins are insect-group specific.
The toxin is coded by a gene named $cry$. There are a number of them.
For example,the proteins encoded by the genes $cryIAc$ and $cryIIAb$ control the cotton bollworms,and $cryIAc$ controls the corn borer.