DNA replication is the process of making new DNA strand from the old one and this process is semi-conservative. Synthesis of new DNA occurs simultaneously and independently on both strands but in one direction as the DNA polymerase used in the process of elongation of the strand of DNA during replication process proceeds in the $5\mathrm{’}3\mathrm{’}$ direction only. Of the two strands, the one that is continuous and occurs without stopping from one end to the other is called leading strand, while the one that is discontinuous in that it occurs in several small chunks one after another is called lagging strand and each short sequence is called Okazaki fragment. The replication process is bidirectional in that it goes in both ways from the point of origin of replication in the DNA. Meselson and Stahl proved that DNA replication is semi conservative in bacteria by experimenting with E. coli.