Simultaneous DNA synthesis on leading & lagging strands
DNAPol III is a dimeric
that synthesizes both the leading and lagging strands
simultaneously. The process occurs consistent with the
requirement that new strand synthesis always occurs 5'3'.
upper diagram, the leading
strand passes through the polymerase subunit in the 5'3' direction 'right to left', such that
synthesis off the leading strand in the 5'3' direction
occurs continuously towards the replication fork.
Simultaneously, the lagging strand passes through the alternate
polymerase subunit, also in the 5'3' direction,
but in order to do so must enter it in the opposite
orientation. This effectively reverses
orientation of 5'3' synthesis off the lagging strand:
both blue arrows in the upper diagram are oriented 'left to
Now, imagine rotating the lower polymerse subunit in the upper diagram 180o to the left (lower diagram) so that both are in the same orientation. The leading and lagging strands now both pass through the dimeric subunits 'left to right' but in opposite 5'3' directions, and that lagging strand synthesis is directed away from the replication fork, as a series of short Okazaki fragments.
Figures © 1999 by Klug & Cummings; text © 2011 by Steven M. Carr