DNA synthesis on the leading & lagging strands of a DNA molecule

Synthesis occurs by extension of a 5'3' RNA Primer (blue). On the leading strand, the 5'3' direction is directed towards the replication fork: a single primer can be extended indefinitely as the fork opens. On the lagging strand, the 5'3' direction is directed away from the fork: synthesis must be re-primed periodically as more template is exposed. This produces a discontinuous set of molecules called Okazaki Fragments.

Figure © 2012 TA Brown, Introduction to Genetics (1st ed.); additional text © 2014 by Steven M. Carr