Essential Cell Biology 5th edition

DNA Replication
207
that is, the direction in which the replication fork is moving. For that to
be true, one strand would have to be synthesized in the 5ʹ-to-3ʹ direction
and the other in the 3ʹ-to-5ʹ direction.
Does the cell have two types of DNA polymerase, one for each direction?
The answer is no: all DNA polymerases add new subunits only to the 3ʹ
end of a DNA strand (see Figure 6–11A). As a result, a new DNA chain
can be synthesized only in a 5ʹ-to-3ʹ direction. This can easily account
for the synthesis of one of the two strands of DNA at the replication fork,
but what happens on the other? This conundrum is solved by the use of
a “backstitching” maneuver. The DNA strand that appears to grow in the
incorrect 3ʹ-to-5ʹ direction is actually made discontinuously, in successive,
separate, small pieces—with the DNA polymerase moving backward
with respect to the direction of replication-fork movement so that each
new DNA fragment can be polymerized in the 5ʹ-to-3ʹ direction.
The resulting small DNA pieces—called Okazaki fragments after the
pair of biochemists who discovered them—are later joined together to
form a continuous new strand. The DNA strand that is made discontinuously
in this way is called the lagging strand, because the cumbersome
backstitching mechanism imparts a slight delay to its synthesis; the other
strand, which is synthesized continuously, is called the leading strand
(Figure 6–13).
Although they differ in subtle details, the replication forks of all cells,
prokaryotic and eukaryotic, have leading and lagging strands. This common
feature arises from the fact that all DNA polymerases work only in
the 5ʹ-to-3ʹ direction—a restriction that allows DNA polymerase to “check
its work,” as we discuss next.
5′
3′
newly synthesized
strands
5′
3′
5′
3′
parental
DNA helix
direction of replicationfork
movement
ECB5 e6.12/6.12
3′
5′
Figure 6–12 At a replication fork, the
two newly synthesized DNA strands are
of opposite polarities. This is because
the two template strands are oriented in
opposite directions.
DNA Polymerase Is Self-correcting
DNA polymerase is so accurate that it makes only about one error in
every 10 7 nucleotide pairs it copies. This error rate is much lower than
can be explained simply by the accuracy of complementary base-pairing.
Although A-T and C-G are by far the most stable base pairs, other,
less stable base pairs—for example, G-T and C-A—can also be formed.
Such incorrect base pairs are formed much less frequently than correct
ones, but, if allowed to remain, they would result in an accumulation of
Okazaki fragments
5′
3′
leading-strand template
of left-hand fork
5′ 3′ 5′ 3′ 5′
3′
direction of fork movement
lagging-strand template
of right-hand fork
5′
most recently
3′
synthesized DNA
3′ 5′
lagging-strand template
of left-hand fork
leading-strand template
of right-hand fork
3′
5′
Figure 6–13 At each replication fork, the
lagging DNA strand is synthesized in
pieces. Because both of the new strands
at a replication fork are synthesized in the
5ʹ-to-3ʹ direction, the lagging strand of
DNA must be made initially as a series of
short DNA strands, which are later joined
together. The upper diagram shows two
replication forks moving in opposite
directions; the lower diagram shows the
same forks a short time later. To replicate
the lagging strand, DNA polymerase uses
a backstitching mechanism: it synthesizes
short pieces of DNA (called Okazaki
fragments) in the 5ʹ-to-3ʹ direction and then
moves back along the template strand
(toward the fork) before synthesizing the
next fragment.