n. (Biochem.) An enzyme which catalyzes creation of a covalent bond between two substrates, resulting in a larger product which is a combination of the two starting materials; especially, DNA ligase, an enzyme which creates a phosphate bond between 5′ and 3′ ends of a DNA chain, resulting in a longer chain of DNA. DNA ligase is important in normal biosynthesis of DNA in the replication cycle of cells. [ PJC ]