A replication fork is a structure formed during DNA replication when specific enzymes (i.e., helicases) separate both DNA strands at the origin of replication. DNA is always synthesized in the 5' to 3' direction, thereby DNA can be synthesized continuously on the leading strand, because the growth of this strand proceeds in the same direction as the movement of the replication fork, while DNA synthesized in several short segments on the lagging strand which are called 'Okazaki fragments'. DNA polymerases are enzymes that can only add nucleotides to the 3' end of a DNA strand, thereby they require the synthesis of short stretches of RNA or 'RNA primers', which are necessary for DNA replication of the lagging strand.
