Synthesis of DNA by DNA polymerases occurs only in 5′→3′ direction. One strand called leading strand, is copied in the same direction as the unwinding helix. The other strand is known as lagging strand. Replication of lagging strand is in a discontinuous way, and in the direction of growth of lagging stand is 3′→5′ though in short segments of DNA which are always in the 5′→3′ direction. These short segments are called Okazaki fragments joined together by the action of DNA ligase.