The essential event in cloning is ligation of the foreign DNA to the vector. Ligation is achieved by using the enzyme DNA ligase. It seals the breaks in the phosphate-sugar backbone of DNA that joins the Okazaki fragments to produce a continuous strand.
In order to catalyse the reaction of a 3'-OH and a 5'-P on double stranded DNA to develop a phosphodiester bond, the ligase needs ATP and magnesium ions. The ligation among the cohesive ended molecules is greatly more effective than the ligation between the blunt ended molecules. For this reason, when ligating blunt ended molecules, the DNA and ligase application should be higher than when ligating the cohesive ended molecules. If the enzyme ligase is not added to cloning process, the vector and cloned DNA would just fall separately.