iGen3 03-08

Leading & Lagging strand synthesis uses a single, dimeric DNAPol III enzyme

    Models of DNA synthesis often show it as occurring independently on the leading and lagging strands, with separate DNAPol IIIs on each. However, replication at any one fork is under the control of a single, dimeric DNAPol III holoenzyme [the two donut-shaped rings] that replicates both parental DNA strands simultaneously. The process occurs consistent with the requirement that new strand synthesis always occurs 5'3'.

    Synthesis off the leading strand (below, blue) occurs in the 5'3' direction, which is oriented towards the replication fork (lower DNA molecule, red strand).To achieve the same orientation on the lagging strand, the lagging strand loops around the subunit  (above, blue). This allows either parental strands to enter the alternate subunits of the polymerase in the same 5'-3' orientation as well as the same right-to-left direction. It also means that lagging strand synthesis will trail a series of Okazaki fragments as succesive segments of the parental strand pass through the polymerase.

    [If you imagine the top subunit rotated 180o to the left, the Okazaki fragments will be directed away from the replication fork, in the manner seen in the simplified diagrams]

Figure © 2010 PJ Russell, iGenetics 3rd ed.; all text material © 2011 by Steven M. Carr