The role of DNA ligase in the construction of a recmobinant 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

Answer

### Step-by-Step Solution:

1. **Understanding DNA Ligase**:
- DNA ligase is an enzyme known for its role in joining DNA fragments. It is often referred to as "molecular glue" or "sealing enzyme" due to its function in creating bonds between DNA segments.

2. **Classification of DNA Ligase**:
- According to the International Union of Biochemistry (IUB) classification, DNA ligase falls under class 6, which includes ligases. This classification helps in understanding its enzymatic function.

3. **Function in Recombinant DNA Construction**:
- In constructing recombinant DNA molecules, DNA ligase is crucial as it facilitates the bonding of two DNA fragments. This bonding occurs through the formation of a specific type of bond known as a phosphodiester bond.

4. **Formation of Phosphodiester Bonds**:
- The phosphodiester bond is formed between the 3' carbon of one DNA fragment and the 5' carbon of another. This bond is essential for linking the sugar-phosphate backbone of the DNA strands.

5. **Evaluating Options**:
- The first option states that DNA ligase is responsible for the formation of phosphodiester bonds between two DNA fragments. This is the correct option.
- The second option mentions the formation of hydrogen bonds between sticky ends of DNA fragments, which is incorrect because DNA ligase does not form hydrogen bonds.
- The third option refers to the ligation of purine and pyrimidine bases, which is also incorrect as DNA ligase does not ligate nitrogenous bases; it ligates DNA fragments.

6. **Conclusion**:
- The correct 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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