DNA ligase that seals nicks in double-stranded DNA during DNA replication, DNA recombination and DNA repair. ATP + (deoxyribonucleotide)n-3'-hydroxyl + 5'-phospho-(deoxyribonucleotide)m = (deoxyribonucleotide)n+m + AMP + diphosphate. Interacts with PCNA. Belongs to the ATP-dependent DNA ligase family. LIG1 mutation db DNA ligase entry