The role of DNA ligase in the construction of a recombinant DNA molecule is

Options

A. formation of phosphodiester bond between two DNA fragments.
B. formation of hydrogen bonds between sticky ends of DNA fragments.
C. ligation of all purine and pyrimidine bases.
D. None of the above

Explanation

**Step-by-Step Solution:**

1. **Understanding Recombinant DNA**: The first step in answering the question is to understand what recombinant DNA (r-DNA) is. Recombinant DNA is formed by combining DNA from two different sources. This process often involves inserting a desired DNA fragment into a target DNA molecule.

2. **Role of DNA Ligase**: DNA ligase is an enzyme that plays a crucial role in the construction of recombinant DNA. After the desired DNA fragment has been inserted into the target DNA, there are often gaps or nicks in the sugar-phosphate backbone of the DNA strands.

3. **Joining DNA Fragments**: The primary function of DNA ligase is to join these DNA fragments together. It does this by catalyzing the formation of phosphodiester bonds between the 3' hydroxyl group of one DNA fragment and the 5' phosphate group of another.

4. **Identifying the Correct Option**: Given the options provided:
- Formation of phosphodiester bond between two DNA fragments (this is correct).
- Formation of hydrogen bond between sticky ends of DNA fragment (this is incorrect; hydrogen bonds are involved in base pairing, not in joining fragments).
- Ligation of all purines and pyrimidines (this is incorrect; ligase does not ligate bases).
- None of the above (this is incorrect as the first option is correct).

5. **Conclusion**: Therefore, the correct answer is that the role of DNA ligase in the construction of a recombinant DNA molecule is the formation of phosphodiester bonds between two DNA fragments.

**Final Answer**: The role of DNA ligase in the construction of a recombinant DNA molecule is the formation of phosphodiester bonds between two DNA fragments.

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