Which of the following statements regarding the structure of proinsulin and mature insulin are not correct ?
(i) Proinsulin is made up of three polypeptide chains -A,B and C
(ii) c- polypeptide chain with 33 amino acids is removed prior to insulin formation
(iii) Mature insulin is made up of 51 amino acids arranged in two polypeptide chain -A and B
(iv) Polypeptide chain A has 30 amino acids and polypeptide chain B has 21 amino acids
(v) Polypeptide chains A and B are interconnected by only one S-S linkage.

Options

A. (i) and (ii)
B. (iii) and (iv)
C. (iv) and (v)
D. (iii),(iv) and (v)

Explanation

To determine which statements regarding the structure of proinsulin and mature insulin are incorrect, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement (i): Proinsulin is made up of three polypeptide chains - A, B, and C.**
- **Analysis:** Proinsulin actually consists of two polypeptide chains (A and B) and a connecting C-peptide. Thus, it is incorrect to say that proinsulin is made up of three polypeptide chains; the C-peptide is not a chain but a part of the proinsulin structure.
- **Conclusion:** This statement is **not correct**.

2. **Statement (ii): C-polypeptide chain with 33 amino acids is removed prior to insulin formation.**
- **Analysis:** The C-peptide is indeed removed during the conversion of proinsulin to mature insulin, but it is not accurate to refer to it as a "polypeptide chain." The C-peptide is a stretch of amino acids, not a separate chain. Therefore, this statement is misleading.
- **Conclusion:** This statement is **not correct**.

3. **Statement (iii): Mature insulin is made up of 51 amino acids arranged in two polypeptide chains - A and B.**
- **Analysis:** This statement is true. Mature insulin consists of 51 amino acids total, with 21 amino acids in the A chain and 30 amino acids in the B chain.
- **Conclusion:** This statement is **correct**.

4. **Statement (iv): Polypeptide chain A has 30 amino acids and polypeptide chain B has 21 amino acids.**
- **Analysis:** This statement is incorrect. The A chain has 21 amino acids, and the B chain has 30 amino acids. Therefore, this statement misrepresents the number of amino acids in each chain.
- **Conclusion:** This statement is **not correct**.

5. **Statement (v): Polypeptide chains A and B are interconnected by only one S-S linkage.**
- **Analysis:** This statement is also incorrect. The two chains are interconnected by two disulfide (S-S) linkages, not just one.
- **Conclusion:** This statement is **not correct**.

### Summary of Incorrect Statements:
The statements that are not correct are:
- (i) Proinsulin is made up of three polypeptide chains - A, B, and C.
- (ii) C-polypeptide chain with 33 amino acids is removed prior to insulin formation.
- (iv) Polypeptide chain A has 30 amino acids and polypeptide chain B has 21 amino acids.
- (v) Polypeptide chains A and B are interconnected by only one S-S linkage.

### Final Answer:
The incorrect statements are (i), (ii), (iv), and (v).

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