DNA ligase that seals nicks in double-stranded DNA during DNA replication, DNA recombination and DNA repair. Can use both ATP and NAD(+), but NAD(+) may be a preferred nucleotide cofactor. ATP + (deoxyribonucleotide)n-3'-hydroxyl + 5'-phospho-(deoxyribonucleotide)m = (deoxyribonucleotide)n+m + AMP + diphosphate. NAD(+) + (deoxyribonucleotide)n-3'-hydroxyl + 5'-phospho-(deoxyribonucleotide)m = (deoxyribonucleotide)n+m + AMP + beta-nicotinamide D-nucleotide. Not stimulated by Ca(2+), Mn(2+) and Ni(2+). Optimum pH is 7.5. Optimum temperature is 80 degrees Celsius. Belongs to the ATP-dependent DNA ligase family.