DNA strands have a directionality, and the different ends of a single strand are called the "3' end" and the "5' end." These terms refer to the carbon atom in ribose to which the next phosphate in the chain attaches. In addition to being complementary, the two strands of DNA are antiparallel: they are orientated in opposite directions. This directionality has consequences in DNA synthesis, because DNA polymerase can only synthesize DNA in one direction by adding nucleotides to the 3' end of a DNA strand.
in DNA replication, the leading strand is defined as the new DNA strand at the replication fork that is synthesized in the 5'→3' direction in a continuous manner.
on the other hand ,The lagging strand is the DNA strand at the opposite side of the replication fork from the leading strand, running in the 3' to 5' direction. Because DNA polymerase III cannot synthesize in the 3'→5' direction, the lagging strand is synthesized in short segments known as Okazaki fragments.
