Biotechnological Applications In Medicine

When human insulin was produced using recombinant DNA technology by Eli Lilly, the A-chain and B-chain were synthesised as separate polypeptides in Escherichia coli and then chemically combined by creating disulphide bonds between them, a process highlighted in NCERT Class 12, Chapter 11 (Biotechnology and its Applications). These disulphide linkages, formed between cysteine residues, hold the two chains together to form the mature, functional hormone, mimicking the natural structure of insulin. Peptide bonds link amino acids within each individual chain, but they do not join the two separate chains together, so that option is incorrect. Hydrogen bonds contribute to the folding and secondary structure of proteins but are too weak to be the defining covalent connection between the insulin chains. Phosphodiester bonds join nucleotides in DNA and RNA and have nothing to do with linking protein chains. Producing the two chains separately in bacteria and then forging these inter-chain bridges was a clever solution to the problem that bacteria cannot themselves carry out the precise processing of the human proinsulin precursor. This made large-scale, reliable manufacture of human insulin possible. A consistency check confirms the answer: the stable covalent connection between two separate polypeptide chains in insulin is specifically achieved through disulphide bonds.