DNA ligase
/ LY-gace /
After the lagging strand has been built as a row of separate Okazaki fragments, it is still not one continuous strand — there are tiny breaks between the pieces, like a wall of bricks laid out but not yet mortared. DNA ligase is the enzyme that mortars the joints: it seals the gaps so the fragments become a single, unbroken strand.
More precisely, ligase joins a nick — a place where two adjacent nucleotides sit side by side but are not chemically bonded, with a free 3'-OH on one side and a 5'-phosphate on the other. Using energy (from ATP in many organisms, or NAD+ in bacteria), ligase forms a phosphodiester bond between them, the same kind of bond that holds the rest of the backbone together. Once it does this at every junction, the once-fragmented strand is mended into one piece.
Ligase is essential not only in replication but throughout DNA biology: it provides the final sealing step in many repair pathways too, wherever a backbone break must be closed. It is also a workhorse of biotechnology — in the lab, DNA ligase is what joins a cut piece of DNA into a vector to make recombinant DNA, literally gluing genes together. The same chemistry that finishes the lagging strand lets us build new DNA molecules by design.
Two Okazaki fragments meet: ...A G C T-3'OH and (phosphate)5'-T G C A... lie next to each other on the same template but are not joined. Ligase forms one phosphodiester bond across that nick, and the two pieces become one strand. The same enzyme glues a gene into a plasmid in cloning.
Ligase forms one backbone bond to seal a nick.
Ligase seals a nick (one missing bond on one strand); it cannot bridge a gap where nucleotides are missing — the gap must first be filled in with DNA by a polymerase before ligase can finish the job.