The reason because replication is carried in the two strands of DNA, is due to in each of them, the RNA polymerase acts in differents ways; while in the 5' - 3' strand, it's replicated continuosly being the leading strand, in the 3' - 5' strand, it's replicated discontinuosly, beomg the lagged strand. This allows the cell to be more efficient in the production of the needed proteins, since these RNA fragments would eventually be used in the formation of aminoacids by the intervention of ribosomes.