Final answer:
DNA ligase catalyzes the formation of a phosphodiester bond to seal nicks in DNA during replication and repair, joining the 3'-OH end of one DNA fragment to the 5' phosphate end of another.
Step-by-step explanation:
DNA ligase uses the phosphodiester bond to seal a nick in the DNA strand during various cellular processes, including DNA replication and repair. After the removal of an incorrect or damaged base by a DNA glycosylase and subsequent actions by AP endonuclease and phosphodiesterase, the 'base-less' sugar phosphate is removed. Then, DNA polymerase adds the correct nucleotide to the 3' end of the nick. Finally, the DNA ligase enzyme catalyzes the formation of a covalent phosphodiester linkage between the 3'-OH end of one DNA fragment and the 5' phosphate end of another DNA fragment, effectively 'sealing' the nick and restoring the integrity of the DNA molecule.
This function of DNA ligase is essential during DNA replication where it helps in joining Okazaki fragments on the lagging strand by forming a bond between the 3' end of one fragment and the 5' end of the next. It is also crucial in DNA repair mechanisms, such as when UV exposure causes pyrimidine dimers, or during the repair of double-stranded breaks that can result in blunt or staggered ends.