Recombinant DNA & Gene Expression in Bacteria — NEET Biology

Correct answer: D — 2, 5, 1, 3, 4, 6

Let's arrange the steps in chronological order: Natural maturation process: - **2. Synthesis of pro-hormone (pro-insulin) with C-peptide.** In humans, insulin is initially synthesized as pro-insulin. - **5. Enzymatic removal of C-peptide from pro-insulin.** The C-peptide is removed during maturation to form active insulin. - **1. Formation of disulfide bridges between A and B chains.** These bridges link the A and B chains in the mature insulin molecule. Recombinant DNA technology process (as done by Eli Lilly): - **3. Introduction of DNA sequences for A and B chains into E. coli plasmids.** This is the first step in bacterial production. - **4. Separate production and extraction of A and B polypeptide chains in bacteria.** E. coli produces the individual chains. - **6. Combination of separately produced A and B chains by creating disulfide bonds in vitro.** This is done to form the functional human insulin. Combining these, the sequence that reflects both natural maturation and recombinant production steps is 2, 5, 1 (natural maturation resulting in active insulin structure), followed by the recombinant production steps 3, 4, 6 (which mimics the active insulin structure). Option A provides a logical sequence: natural pro-insulin synthesis, C-peptide removal, disulfide bond formation (for natural and also relevant for final structure). Then the recombinant steps: gene introduction, separate chain production, and in vitro combination. This order correctly mixes the processes to show the overall development. Option B and D mix the steps illogically. Option C incorrectly places C-peptide removal after disulfide bridge formation, which is not correct as C-peptide is removed before the final active structure is formed. The question tests the understanding of both the natural process of insulin maturation and the specific steps involved in its recombinant production.