Insulin Synthesis Location

The question asks about the synthesis and structure of insulin. Insulin is a protein hormone that is secreted from the cell. Proteins destined for secretion, insertion into membranes, or delivery to certain organelles like lysosomes are typically synthesized on ribosomes attached to the endoplasmic reticulum, specifically the rough endoplasmic reticulum (RER).

• Statement A says insulin is synthesized in the rough endoplasmic reticulum of the B cells of islets of Langerhans. This is consistent with the synthesis pathway for secreted proteins.
• Statement B says insulin is synthesized in the cytosol on free ribosomes. Proteins synthesized on free ribosomes usually function within the cytosol, nucleus, or other organelles like mitochondria or peroxisomes, but not typically secreted proteins like insulin.

Therefore, statement A is correct, and statement B is incorrect.

Insulin Structure and Disulfide Bridges

Insulin is initially synthesized as a single polypeptide chain called proinsulin, which is then cleaved to form mature insulin. Mature human insulin consists of two polypeptide chains, Chain A and Chain B, linked by disulfide bonds. This makes it an AB heterodimer structure. The A chain has 21 amino acids, and the B chain has 30 amino acids.

The disulfide bridges in mature human insulin are crucial for its correct folding and function. There are three disulfide bridges:

• One intrachain disulfide bridge within the A chain.
• Two interchain disulfide bridges connecting the A and B chains.

Let's look at the statements regarding the specific disulfide bridges:

• Statement C describes an intrachain bridge A8‐A13 and two interchain bridges A6‐B10 and A21‐B18.
• Statement D describes an intrachain bridge A6‐A11 and two interchain bridges A7‐B7 and A20‐B19.

The correct disulfide bridge locations in human insulin are A6‐A11 (intrachain), A7‐B7 (interchain), and A20‐B19 (interchain). Thus, statement D accurately describes the disulfide bridges, while statement C does not.

Therefore, statement D is correct, and statement C is incorrect.

Insulin Gene Location and Structure

The gene that codes for insulin is located on a specific chromosome in humans. Genes also contain coding regions (exons) and non-coding regions (introns) that are removed during mRNA processing.

Let's examine the statements about the insulin gene:

• Statement E says the insulin gene is on the long arm of chromosome 11 and has two introns and three exons.
• Statement F says the insulin gene is on the short arm of chromosome 11 and has two introns and three exons.

The human insulin gene is located on chromosome 11, specifically on the short arm (p arm). The gene structure typically consists of 3 exons interrupted by 2 introns. Therefore, the description of the gene structure (two introns and three exons) is correct in both E and F, but the location on the short arm makes statement F correct.

Therefore, statement F is correct, and statement E is incorrect regarding the chromosome arm.

Correct Combination of Statements

Based on the analysis:

• Statement A is correct (synthesis in RER).
• Statement D is correct (correct disulfide bridges A6‐A11, A7‐B7, A20‐B19).
• Statement F is correct (gene on short arm of chromosome 11 with two introns and three exons).

The combination of correct statements is A, D, and F.