Leading &
Lagging strand DNA 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 [shown as 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'.
The chromosomal
DNA is oriented so that the parental DNA strand
[blue] is 'unzipped' left to right by the DNA
helicase. The DNAPol sits at the
replication fork and moves continuously to the right.
Synthesis of a new copy [below, red] of the leading strand [below,
blue] occurs in the 5'3' direction. Synthesis is continuous, because fork
movement always passes more of the leading strand through the
lower DNAPol subunit.
Synthesis in the
5'3'
direction in
the same physical orientation off the other,
lagging strand [above, blue] requires that it loops
around the upper DNAPol subunit
[above, blue]. This
allows both anti-parallel 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.
DNA synthesis
of the lagging strand is necessarily discontinuous,
because synthesis must be constantly re-initiated by addition
of an RNA primer [green] as more of the exposed
parental strand passes through the DNAPol. This
creates a series of Okazaki fragments beginning with
an RNA primer and ending when new synthesis encounters
the 5' end of the previous fragment. The primers are
subsequently removed and the gap 'repaired' by a different DNA
polymerase