DNA ligase
/ D-N-A LY-gayss /
Imagine you have laid a paved path as a row of stones placed end to end, but the mortar between some of them is missing, so there are tiny breaks. The path looks complete, yet step on a gap and it gives way. DNA ligase is the cell's mortar-layer: it finds the breaks left in the sugar-phosphate backbone of DNA and seals them, turning a row of separate pieces into one solid, continuous strand.
Specifically, DNA ligase repairs nicks — places where two stretches of DNA sit right next to each other but their backbones are not yet chemically joined. It catalyzes the formation of the bond that links them, using a little energy to do so. Its most famous routine job is on the lagging strand: after the RNA primers of the Okazaki fragments are removed and replaced with DNA, the fragments still have nicks between them, and ligase stitches them into one unbroken strand. It is the final 'finisher' that completes the lagging strand.
DNA ligase matters far beyond replication. The same kind of break-sealing is the last step of many DNA repair pathways, so ligase is essential for keeping the genome intact after damage. In the laboratory it is a workhorse of genetic engineering, used to glue together pieces of DNA from different sources. A common misconception is that DNA polymerase finishes the job by itself; in reality polymerase can fill a gap but cannot seal the final break between the new piece and the next — only ligase forms that closing bond.
In gene cloning, scientists cut DNA with enzymes to make matching ends, mix a gene with a circular carrier DNA, and add DNA ligase. The ligase seals the gene into the carrier, producing a single closed loop ready to be put into cells — the same sealing chemistry the enzyme performs naturally on Okazaki fragments.
DNA ligase seals backbone breaks, finishing the strand and many repairs.
Ligase seals only the backbone break between adjacent pieces — it cannot synthesize new DNA, so it always works after polymerase has filled the gap.