DNA ligase that seals nicks in double-stranded DNA during DNA replication, DNA recombination and DNA repair. Has also low activity with dATP. Inactive with NAD(+), CTP, GTP, UTP, dCTP, dGTP or dTTP. ATP + (deoxyribonucleotide)n-3'-hydroxyl + 5'-phospho-(deoxyribonucleotide)m = (deoxyribonucleotide)n+m + AMP + diphosphate. Optimum pH is 6.0-7.0. Optimum temperature is 60-80 degrees Celsius. Belongs to the ATP-dependent DNA ligase family.