The alpha isoform interacts with DNA-repair protein XRCC1 and can correct defective DNA strand-break repair and sister chromatid exchange following treatment with ionizing radiation and alkylating agents. The beta isoform does not interact with XRCC1 and may be specifically involved in the completion of homologous recombination events that occur during meiotic prophase. ATP + (deoxyribonucleotide)n-3'-hydroxyl + 5'-phospho-(deoxyribonucleotide)m = (deoxyribonucleotide)n+m + AMP + diphosphate. Interacts (via BRCT domain) with the nuclear DNA-repair protein XRCC1. The alpha isoform is expressed in all tissues, while the beta isoform is expressed only in the testis. During male germ cell differentiation, expression of the beta isoform begins in the later stages of meiotic prophase and ends in the round spermatid stage. The PARP-type zinc finger is required for DNA ligase activity. Belongs to the ATP-dependent DNA ligase family.