What enzyme joins Okazaki fragments together?

The enzyme that joins Okazaki fragments together is DNA ligase.

During replication, DNA polymerase leaves adjacent Okazaki fragments separated by small breaks, or nicks, in the sugar-phosphate backbone. Once RNA primers have been removed and the gaps filled with DNA, DNA ligase completes the repair by forming a phosphodiester bond between neighboring nucleotides. This converts the discontinuous pieces into one continuous daughter strand.

Ligase does not synthesize long stretches of DNA, unwind the double helix, or remove nucleotides. Those jobs belong mainly to DNA polymerase, helicase, and nucleases or specialized primer-removal enzymes. A useful distinction is that ligase seals a pre-existing nick, whereas polymerase adds nucleotides to a free 3′ hydroxyl group.

The basic ligation reaction requires energy. Bacterial DNA ligases commonly use NAD+ or ATP, while eukaryotic ligases use ATP. In human nuclear DNA replication, DNA ligase I is especially important for joining Okazaki fragments after primer processing. Failure to seal these nicks can leave fragile DNA intermediates and contribute to chromosome breaks, making ligase essential for genome stability.