Correct option is D
Explanation-
Statement A - It is synthesized in rough endoplasmic reticulum of the B cells of islets of Langerhans - Correct
Protein synthesis in eukaryotic cells begins on ribosomes. For secretory proteins like insulin, the synthesis begins on free ribosomes, but as the signal peptide emerges, the ribosome attaches to the rough endoplasmic reticulum (RER).
Preproinsulin is first synthesized in the RER, where it undergoes folding and disulfide bond formation. It is then converted to proinsulin, which is transported to the Golgi and processed into insulin and C-peptide.
Statement B - It is synthesized in cytosol on free ribosomes of the B cells of islets of Langerhans - Incorrect
While initial translation begins in the cytosol, proteins like insulin that are destined for secretion are completed in the RER. Insulin is not a cytosolic protein; it must be processed and packaged in secretory vesicles, so its synthesis is completed on ribosomes bound to the RER, not free ribosomes.
Statement C - Insulin has an AB heterodimer structure with one intrachain (A8–A13) and two interchain disulfide bridges (A6–B10 and A21–B18) - Incorrect
This statement contains incorrect disulfide bond positions.
The correct disulfide bonds are:
One intrachain disulfide bond within the A chain: between A6 and A11
Two interchain disulfide bonds between A and B chains : A7 – B7 and A20 – B19
Statement D - Insulin has an AB heterodimer structure with one intrachain (A6–A11) and two interchain disulfide bridges (A7–B7 and A20–B19) - Correct
This is the accurate structural description of insulin.
Insulin is made up of - A chain (21 amino acids) and B chain (30 amino acids)
These are linked by disulfide bonds:
Intrachain: A6–A11
Interchain: A7–B7 and A20–B19
Statement E - The gene for insulin is located on the long arm of chromosome 11 and has two introns and three exons - Incorrect
The insulin (INS) gene is located on the short arm of chromosome 11, specifically at 11p15.5, not the long arm. It does have two introns and three exons, but the chromosomal arm is wrongly stated, making this statement incorrect.
Statement F - The gene for insulin is located on the short arm of chromosome 11 that has two introns and three exons - Correct
This is accurate.
Chromosomal location: 11p15.5 (short arm of chromosome 11)
Gene structure: two introns and three exons
Final Answer:
The correct combination of statements is - A, D, and F (option d )
