Synthesis of $DNA$ by $DNA$ polymerases occurs only in $5' \rightarrow 3'$ 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 $3'\rightarrow 5'$ though in short segments of $DNA$ which are always in the $5' \rightarrow 3'$ direction. These short segments are called Okazaki fragments joined together by the action of $DNA$ ligase.