Eli Lilly, an American company, prepared two DNA sequences corresponding to A and B chains of human insulin and introduced them in the plasmids of E.coli to produce insulin chains. Chains A and B were produced separately, extracted, and combined by creating

Options

A. peptide bonds
B. ionic bonds
C. H-bonds
D. disulfide bonds

Answer

To solve the question regarding the production of insulin chains A and B by Eli Lilly, we will follow these steps:

### Step-by-Step Solution:

1. **Understanding the Context**: Eli Lilly produced two DNA sequences that correspond to the A and B chains of human insulin. These sequences were introduced into plasmids of E. coli, a common bacterium used in biotechnology for protein production.

**Hint**: Think about the role of plasmids in genetic engineering.

2. **Production of Chains**: The A and B chains of insulin were produced separately within the E. coli cells. This is a standard procedure in recombinant DNA technology where proteins are expressed in bacterial systems.

**Hint**: Consider why it might be beneficial to produce the chains separately.

3. **Extraction of Chains**: After the chains were produced, they were extracted from the bacterial cells. This is necessary to purify the proteins before they can be combined.

**Hint**: Reflect on the importance of purification in protein production.

4. **Combining the Chains**: The question states that chains A and B were combined after extraction. The correct method for combining these two chains to form functional insulin involves the formation of specific types of bonds.

5. **Identifying the Type of Bond**: The options provided are:
- A) Peptide bonds
- B) Ionic bonds
- C) Hydrogen bonds
- D) Disulfide bonds

In the case of insulin, the A and B chains are connected by disulfide bonds, which are covalent bonds formed between the sulfur atoms of cysteine residues in the two chains.

**Hint**: Recall the structure of insulin and the role of disulfide bonds in stabilizing protein structure.

6. **Conclusion**: Therefore, the correct answer is option D, which states that chains A and B were combined by creating disulfide bonds.

### Final Answer:
The correct answer is **D) Disulfide bonds**.