(Science: enzyme molecular biology) enzyme involved in dna replication. The dna ligase of E. Coli seals nicks in one strand of double stranded dna, a reaction required for linking precursor fragments during discontinuous synthesis on the lagging strand. Nicks are breaks in the phosphodiester linkage that leave a free 3_ OH and 5_ phosphate. The ligase from phage t4 has the additional property of joining two dna molecules having completely base paired ends. Dna ligases are crucial in joining dna molecules and preparing radioactive probes (by nick translation) in recombinant DNA Technology. An enzyme that promotes the joining up of adjacent bonds in a polynucleotide Chain, which is used in [[gene.