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1. DNA ligase is an enzyme that catalyzes the formation of a phosphodiester bond between two DNA strands joining them together. DNA ligase is present in eukaryotes and prokaryotes and is involved in the DNA repair processes of single- or double-strand breaks.
DNA polymerase I is an enzyme that forms of a phosphodiester bond between DNA and deoxyribonucleotides providing the "step-by-step" synthesis of DNA. This enzyme is produced in prokaryotes and is involved in the excision repair and processing of Okazaki fragments during replication. The enzyme possesses polymerase activity, as well as 3'–5' and 5'–3' exonuclease activities.
DNA polymerase delta also forms of a phosphodiester bond between DNA and deoxyribonucleotides. The enzyme is produced in eukaryotes and is the main polymerase involved in DNA replication. It is also used in DNA repair processes. In contrast to DNA ligase and bacterial DNA polymerase I, DNA polymerase delta consists of several subunits.
2. As DNA ligase requires PCNA free from Polymerase delta, Okazaki fragments are joined by ligase after the end of the elongation and polymerase delta dissociation.