the specific method for producing genetically engineered human insulin (developed by Eli Lilly in 1983) involved preparing two DNA sequences corresponding to chains A and B, introducing them into E. coli plasmids to produce the chains separately, and then extracting and combining them by creating disulphide bonds . Other statements often associated with this question (regarding C-peptide in pro-hormone) describe natural insulin synthesis, not the specific genetically engineered product which consists of separate chains combined later.