Which enzyme seals nicks between Okazaki fragments on the lagging strand?

Which enzyme seals nicks between Okazaki fragments on the lagging strand?

• A. DNA polymerase III
• B. DNA Ligase
• C. Primase
• D. Topoisomerase

Answer: (B)

Sealing nicks between Okazaki fragments on the lagging strand requires a specialized enzyme that forms the final backbone connection. On the lagging strand, synthesis occurs in short segments called Okazaki fragments, each started by an RNA primer laid down by primase. DNA polymerase III extends each fragment, but after primers are removed and replaced with DNA, there’s a nick between fragments that must be joined to produce a continuous strand. DNA ligase performs this job by creating the phosphodiester bond that links the 3′ end of one fragment to the 5′ end of the next, completing the sugar-phosphate backbone. In bacteria, ligase uses NAD+ as a cofactor (in eukaryotes it uses ATP) to drive this bond formation. Without ligase, the fragments would remain separate and the genome wouldn’t be fully connected. The other enzymes have different roles: polymerase III builds the DNA, primase provides the RNA primer, and topoisomerase relieves supercoiling ahead of the fork.

Sealing nicks between Okazaki fragments on the lagging strand requires a specialized enzyme that forms the final backbone connection. On the lagging strand, synthesis occurs in short segments called Okazaki fragments, each started by an RNA primer laid down by primase. DNA polymerase III extends each fragment, but after primers are removed and replaced with DNA, there’s a nick between fragments that must be joined to produce a continuous strand. DNA ligase performs this job by creating the phosphodiester bond that links the 3′ end of one fragment to the 5′ end of the next, completing the sugar-phosphate backbone. In bacteria, ligase uses NAD+ as a cofactor (in eukaryotes it uses ATP) to drive this bond formation. Without ligase, the fragments would remain separate and the genome wouldn’t be fully connected.

The other enzymes have different roles: polymerase III builds the DNA, primase provides the RNA primer, and topoisomerase relieves supercoiling ahead of the fork.