DNA ligase joins Okazaki fragments on a newly copied DNA strand.
During DNA replication, the two template strands are copied in opposite directions because DNA polymerases extend a new strand only from its 5′ end toward its 3′ end. The continuously copied strand is the leading strand; the lagging strand is made in short pieces called Okazaki fragments.
DNA ligase seals the remaining breaks between adjacent fragments by creating phosphodiester bonds in the sugar-phosphate backbone. Before ligase acts, RNA primers are removed and the resulting gaps are filled with DNA. The enzyme therefore performs a final “joining” step rather than adding most of the new sequence.
Ligases are also important in DNA repair and biotechnology. A common laboratory enzyme, T4 DNA ligase, joins compatible DNA ends and is widely used in cloning experiments. Ligase should not be confused with helicase, which separates the two DNA strands.