in 1968 okasaki and co-workers discovered that much of the most recently synthesized DNA was present transiently as small DNA fragment. these fragments subsequently became joined to the growing DNA strand(okasaki et al.,1968). This finding of "discontinuous synthesis" immediately provided an answer as to how DNA polymerases which act only in a 5'-3' direction can also bring about net synthesis in 3'-5' direction. As the DNA strands unwind ahead of the replication fork, single-stranded regions of DNA are exposed. Synthesis of the so-called leading strand in the 5'-3' direction can occur continuously, simply by elongation of the growing chain. However' synthesis on the other "lagging" strand in the 3'-5' direction can be brought about by successive synthesis of small fragments in a backward (5'-3') direction as shown in fFig. 6. the size of these small "okasaki fragments" is about 1000 nucleotides in bacteria, but only 100-200 in mammalian cells.