T4 DNA Ligase catalyzes the formation of a phosphodiester bond between juxtaposed 5’-phosphate and 3’-hydroxyl termini in duplex DNA or RNA. Catalyzes the joining of duplex DNA molecules at blunt ends. Covalently joins DNA fragments with complementary cohesive ends. Seals single-stranded nicks in duplex DNA, RNA or DNA/RNA hybrids. Suitable for cloning of restriction fragments and joining linkers or adapters to blunt-ended DNA.