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 statements regarding insulin, let's analyze each option step-by-step:

1. Statement (a): C-peptide is not present in mature insulin.
Insulin is synthesized as a prohormone (pro-insulin) which contains an extra stretch of amino acids called the C-peptide (connecting peptide). During maturation, this C-peptide is cleaved off, leaving only the active A and B chains. Therefore, mature insulin does not contain C-peptide. This statement is correct.

2. Statement (b): The insulin produced by rDNA technology has C-peptide.
Genetically engineered insulin (such as Humulin, produced using recombinant DNA technology) is synthesized by separately producing A and B chains in Escherichia coli and then joining them via disulfide bonds. This process bypasses the need for pro-insulin processing, so the final rDNA-produced insulin does not contain C-peptide. This statement is incorrect.

3. Statement (c): The pro-insulin has C-peptide.
Pro-insulin is the precursor form of insulin that contains the A-chain, B-chain, and the connecting C-peptide. This statement is correct.

4. Statement (d): A-peptide and B-peptide of insulin are interconnected by disulphide bridges.
Mature insulin consists of two polypeptide chains: chain A (with 21 amino acids) and chain B (with 30 amino acids). These two chains are linked together by two inter-chain disulfide bonds (disulphide bridges). This statement is correct.

Comparing our analysis with the choices, the correct statements are (a), (c), and (d). Thus, the correct option is (a), (c) and (d) only.