(i) Insulin is,naturally secreted by pancreas through ffcells of islets of langerhence. Insulin control the blood glucose level. When pancreas failed to produce enough insulin, it causes diabetes. This problem could only be controlled by giving insulin to the body. Initially, insulin gives to a diabetic patient wax extracted from pancreas by killing animals such as cow and pig. This insulin injected to human create allergies other kind of problems. Naturally synthesised pro - insulin made by mammals contain a C - peptide that insulin undergoes maturation and C - peptide separate out when blood glucose level increases. When insulin was produced using rDNA techniques, this separation of C - peptide become a major challange to get mature insulin artificially.

(ii) In 1983, American company Eli Lilly designed two DNA sequence, similar to A and B chain of insulin. E. coli bacteria used to produce A and B chains in culture medium separately. Though disuifide linkage both the chains are exactly like mature insulin. This artifically synthesised insulin is named humulin, which is very safe and cause no ellergy.

(iii) Insulin naturally produced by pancreas through ß - cells of islets of langerhence naturally insulin produced in the form of proisulin having 3 chains A, B and C, chain C is interlinking between A and B chains. When pro - insulin undergoes maturation, C peptide separate out from insulin due to the increased blood level, but insulin produced artifically do not have that C chain because artificial proinsulin can not be produced in such way. Company only produce mature insulin having two A and B chains.