What is the cause of discontinuous synthesis of $DNA$ on one of the parental strands of $DNA$ ? What happens to these short stretches of synthesised $DNA$ ?

The $DNA$-dependent $DNA$ polymerases catalyse polymerisation only in one direction, that is $5^{\prime} \rightarrow$ $3 '$. This creates some additional complications at the replicating fork. Consequently, on leading strand (the template with polarity $\left.3^{\prime} \rightarrow 5^{\prime}\right)$, the replication is continuous, while on the lagging strain (the template with polarity $\left.5^{\prime} \rightarrow 3^{\prime}\right)$, it is discontinuous. The discontinuously synthesised fragments as Okazaki fragments are later joined by the enzyme $DNA$ ligase.