Function of DNA ligase –
**Core Concept**
DNA ligase is a crucial enzyme involved in the process of DNA replication and repair. It plays a vital role in sealing the gaps between nucleotides, thereby ensuring the integrity and stability of the genome.
**Why the Correct Answer is Right**
During DNA replication, the leading strand is synthesized continuously, whereas the lagging strand is synthesized in short, discontinuous segments called Okazaki fragments. These fragments are about 1000-2000 nucleotides long and are synthesized in the 5' to 3' direction. The gaps between these fragments are filled with nucleotides, but the process is not complete until DNA ligase seals the phosphodiester bond between the 3' end of one fragment and the 5' end of the adjacent fragment. This process is essential for maintaining the continuity of the DNA strand and ensuring accurate replication.
**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because DNA polymerase is responsible for adding nucleotides to the growing DNA strand, but it does not seal the gaps between fragments.
* **Option B:** This is incorrect because DNA helicase is an enzyme that unwinds the double helix structure of DNA, allowing other enzymes access to the template strands.
* **Option C:** This is incorrect because topoisomerase is an enzyme that relaxes supercoils in DNA, but it does not seal the gaps between fragments.
**Clinical Pearl / High-Yield Fact**
DNA ligase deficiency can lead to genetic disorders, including Bloom syndrome, which is characterized by short stature, facial abnormalities, and an increased risk of cancer.
**Correct Answer: C. DNA ligase is responsible for sealing the gaps between Okazaki fragments during DNA replication.**