With regard to insulin choose correct options

(a) C-peptide is not present in mature insulin.

(b) The insulin produced by rDNA technology has C-peptide.

(c) The pro-insulin has C-peptide.

(d) A-peptide and B-peptide of insulin are interconnected by disulphide bridges.

To determine the correct options regarding insulin, let us analyze each statement step-by-step:

Statement (a): C-peptide is not present in mature insulin.
Insulin is synthesized in the human body as a precursor called pro-insulin (or prohormone), which contains an extra polypeptide stretch called the C-peptide (connecting peptide). Before insulin is released into the blood as a fully active, mature hormone, this C-peptide is cleaved off and removed. Therefore, mature insulin consists only of A and B chains, and does not contain C-peptide. This statement is correct.

Statement (b): The insulin produced by rDNA technology has C-peptide.
In recombinant DNA (rDNA) technology, insulin (such as Humulin) is produced by synthesizing DNA sequences for the A and B chains separately. These sequences are inserted into plasmids of Escherichia coli to produce A and B chains individually. These chains are then extracted and bonded chemically using disulfide bridges to form mature insulin. This bypasses the need for the C-peptide altogether. Thus, rDNA-produced insulin does not contain C-peptide. This statement is incorrect.

Statement (c): The pro-insulin has C-peptide.
Pro-insulin is the inactive prohormone precursor of insulin. It consists of the A-chain, the B-chain, and the connecting C-peptide. This statement is correct.

Statement (d): A-peptide and B-peptide of insulin are interconnected by disulphide bridges.
Mature human insulin is composed of two polypeptide chains: the A-chain (21 amino acids) and the B-chain (30 amino acids). These two chains are held together and stabilized by disulfide bridges (covalent sulfur-sulfur bonds). Specifically, there are two inter-chain disulfide bonds connecting the A and B chains. This statement is correct.

Conclusion: Since statements (a), (c), and (d) are correct, the correct option is (a), (c) and (d) only.