Waston and Crick had immediately proposed a semi conservative DNA replication scheme for replication DNA. After the completion of replication, each DNA molecule would have one parental and one newly synthesized strand. Later M.S.Meselson and F.W.Stahl (1957-58) confirmed the semi-conservative mode with E. coli growing in isotopic 15 N.
The DNA dependent DNA polymerases catalyse polymerization only in one direction, that is 5`→ 3`. This creates some additional complications at the replicating fork. Consequently, on one strand (the template with polarity 3`→ 5`), the replication is continuous, while on the other (the template with polarity 5`→3`), it is discontinuous. The discontinuously synthesized fragments are later joined by the enzyme ENA ligase. The DNA polymerases on their own cannot initiate the process of replication.