Complete DNA Replication and Storage Practice Test 2026

Sealing nicks to join fragments is the job of DNA ligase, the enzyme that forms a phosphodiester bond between adjacent nucleotides on the same strand. After DNA polymerase fills in gaps and RNA primers are removed on the lagging strand, there are nicks in the sugar-phosphate backbone that must be connected to create a continuous DNA molecule. DNA ligase catalyzes that bond formation, effectively gluing the fragments together. It does this by activating the 5′ phosphate end and linking it to the 3′ hydroxyl end, using energy from ATP (in eukaryotes) or NAD+ (in bacteria). Without ligase, fragments would remain separate pieces rather than a single, continuous strand.

DNA polymerase, by contrast, is the builder that extends new DNA strands but does not seal the backbone. Helicase unwinds the double helix to give access for replication, and primase lays down RNA primers to start synthesis.