Continuous & Discontinuous synthesis

Schematic presentation of Continuous & Discontinuous DNA synthesis

Extension of RNA primers occurs by addition of nucleotides to the 3' ends of both DNA strands simultaneously, such that growth occurs on both in the 5'3' direction only. In this schematic, the replication fork (RF) is shown as opening to the right. On the lower leading strand, synthesis is continuous because extension of a single RNA primer occurs without interruption into the replication fork as it continues to open to the right. On the upper lagging strand, synthesis is discontinuous, since new RNA primers must be added as opening of the replication fork continues to expose new template. This produces a series of disconnected Okazaki fragments.

In this diagram, note that 'left' and 'right', and 'top' and 'bottom' are arbitrary: the molecule could be flipped vertically or horizontally. Only the 5'-3' orientation of the two strands is fixed. Note also that the schematic shows extension on either strand occuring as two separate processes, separated from the RF. In fact, DNA synthesis occurs as a single process involving a dimeric polymerase molecule situated at the RF.