During DNA replication the DNA duplex is unwound and two daughter strands are formed at the replication fork. DNA polymerase can only add nucleotides in the 5' to 3' direction, hence the synthesis of the one of the daughter strands, the leading strand, can proceed continuously in the same direction as movement of the replication fork.
However a complication arises with the replication of the other daughter strand, the lagging strand. As growth must occur in the 5' to 3' direction the copying of the template must somehow occur in the opposite direction as the movement of the replication fork. The cell accomplishes this by synthesising a new RNA primer every hundred bases or so as more of the strand is exposed. Each of these primers is elongated in the 5' to 3' direction forming segments called Okazaki fragments, named after their discoverer Reiji Okazaki. The RNA primer is then removed and the enzyme DNA ligase joins the adjacent fragments together.
- ↑ Pg. 146, Molecular Cell Biology, Harvey Lodish et al.