In DNA replication, which enzyme seals the sugar-phosphate backbone by joining Okazaki fragments on the lagging strand?

• A. DNA ligase seals sugar-phosphate backbone.
• B. Primase seals primers.
• C. Helicase seals Okazaki fragments.
• D. DNA polymerase seals nicks.

Answer: (B)

On the lagging strand, DNA is made in short segments called Okazaki fragments that must be stitched together into one continuous strand. The enzyme that does this final stitching is DNA ligase. It seals the sugar-phosphate backbone by forming a phosphodiester bond between the adjacent fragments, effectively closing the nicks left after primer removal and fragment replacement. This ligation requires energy, provided by ATP in eukaryotes (or NAD+ in prokaryotes).

Primase, by contrast, provides the starting point for synthesis by laying down RNA primers but does not join fragments. Helicase unwinds the DNA ahead of the replication fork, enabling synthesis, and DNA polymerase extends each fragment and fills in gaps, but the actual joining of fragments is accomplished by DNA ligase.