Which enzyme seals nicks between Okazaki fragments?

• A. DNA polymerase
• B. Helicase
• C. Primase
• D. DNA ligase

Answer: (D)

The main concept is how the lagging strand is connected after fragments are made. On the lagging strand, DNA is synthesized in short chunks called Okazaki fragments. Each fragment starts with an RNA primer and is extended by DNA polymerase, but after the primer is removed and the gaps between fragments need to be joined, those nicks must be sealed to create a continuous backbone. DNA ligase is the enzyme that does this: it catalyzes the formation of a phosphodiester bond between adjacent fragments, effectively stitching the fragments together into one continuous strand. Without ligase, the fragments would remain separate pieces with gaps in the sugar-phosphate backbone.

For context, other enzymes have different roles: helicase unwinds the double helix, primase lays down the RNA primers, and DNA polymerase fills in nucleotides and extends DNA, but it doesn’t seal the backbone between fragments.