Recombinant insulin design — Which approach correctly describes a classic method for synthesizing human insulin using recombinant DNA technology?

Introduction / Context:
Recombinant human insulin was produced using early bacterial expression strategies that overcame challenges of peptide stability and processing. Understanding the chain-by-chain method explains how bioidentical insulin was obtained without relying on animal pancreases.

Given Data / Assumptions:

• Insulin comprises A and B polypeptide chains linked by disulfide bonds.
• Bacteria do not process preproinsulin like human beta cells.
• Fusion expression and chemical cleavage enabled high yields and purity.

Concept / Approach:
One classic strategy used two synthetic genes (for A and B chains). Each chain was expressed in E. coli as a β-galactosidase fusion, cleaved with cyanogen bromide, purified, and then oxidatively refolded together to form native disulfide bonds. Alternative approved methods later used proinsulin cDNA with enzymatic processing, but the chain-by-chain method is historically significant and matches the option given.

Step-by-Step Solution:

Verification / Alternative check:
Regulatory and historical accounts of Humulin describe chain-by-chain and proinsulin strategies; both yield bioidentical insulin but the option specifying separate chain synthesis matches the classic approach.

Why Other Options Are Wrong:

• Genomic DNA from islets: includes introns and regulatory sequences unsuitable for direct bacterial expression.
• cDNA without processing: proinsulin requires proper cleavage; expression systems must include processing steps.
• Single synthetic gene with no processing: still requires folding/processing; oversimplified.
• Plasma extraction: not feasible for insulin.

Common Pitfalls:
Assuming bacteria can process preprohormones like human cells; engineered strategies are required to obtain mature insulin.

Final Answer:
Using chemically synthesized DNA sequences for the A and B chains separately (expressed as fusion proteins and then recombined).