Ligases serve as:

Options

A. molecular glue that help in sealing gaps in DNA fragments
B. breaking hydrogen-bond between base pairs
C. uncoiling of DNA strand
D. synthesis of complementary DNA

Answer

### Step-by-Step Solution:

1. **Understanding Ligases**: Ligases are enzymes that play a crucial role in molecular biology, particularly in the processes of DNA replication and repair.

2. **Function of Ligases**: The primary function of ligases is to join two large molecules together. In the context of DNA, they are responsible for sealing gaps between DNA fragments.

3. **Mechanism of Action**: Ligases catalyze the formation of phosphodiester bonds between the phosphate group of one DNA strand and the deoxyribose sugar of another strand. This is essential for maintaining the integrity of the DNA molecule.

4. **Role in DNA Replication**: During DNA replication, especially on the lagging strand, small fragments known as Okazaki fragments are formed. Ligases are responsible for sealing the gaps between these fragments, ensuring that the DNA strand is continuous.

5. **Clarifying Misconceptions**: Ligases do not break hydrogen bonds between base pairs, do not assist in the uncoiling of DNA strands, and do not synthesize complementary DNA. Their primary role is to join DNA fragments.

6. **Conclusion**: Therefore, the correct function of ligases is that they serve as molecular glue that helps in sealing gaps in DNA fragments.